Mail notification fails when LDAP user is removed but still listed as watcher

When an user is removed from LDAP, they are not removed from the watcher list of an issue ticket. This causes JIRA to hit a NPE and not send any notifications to any user.

This bug has become a bigger issue with the new AutoWatch function.

Tested on JIRA 5.1.6

Steps to reproduce:

• Have an issue ticket with working notifications
• Create new user in LDAP, with a valid e-mail address
• Within JIRA, setup a new directory, connecting to LDAP
• Synchronise your LDAP directory
• Login as the new user from LDAP
• Watch an issue ticket
• Remove this user from LDAP
• Perform another Sync, removing this user from JIRA
• Perform an issue action on the 'watched' ticket
• Observe that no notifications are sent (mail queue remains empty)
• Logs show:

  2012-10-10 15:14:49,530 http-6060-16 ERROR admin 914x590x1 vmq9hm 172.28.12.206 /rest/api/2/issue/TEST-1/comment [atlassian.event.internal.EventPublisherImpl] There was an exception thrown trying to dispatch event 'com.atlassian.jira.event.issu
  e.IssueEvent@4b822430[issue=TEST-1,comment=com.atlassian.jira.issue.comments.CommentImpl@28cb7872,worklog=<null>,changelog=<null>,eventTypeId=6,sendMail=true,params={eventsource=action, baseurl=http://localhost:6060},subtasksUpdated=false]' from th
  e invoker 'com.atlassian.event.internal.SingleParameterMethodListenerInvoker@49bcefcf'.java.lang.RuntimeException: java.lang.NullPointerException        at com.atlassian.event.internal.SingleParameterMethodListenerInvoker.invoke(SingleParameterMethodListenerInvoker.java:50)
          at com.atlassian.event.internal.AsynchronousAbleEventDispatcher$2.run(AsynchronousAbleEventDispatcher.java:60)
          at com.atlassian.event.internal.AsynchronousAbleEventDispatcher$1.execute(AsynchronousAbleEventDispatcher.java:28)
          at com.atlassian.event.internal.AsynchronousAbleEventDispatcher.dispatch(AsynchronousAbleEventDispatcher.java:56)
          at com.atlassian.event.internal.EventPublisherImpl.invokeListeners(EventPublisherImpl.java:151)
          at com.atlassian.event.internal.EventPublisherImpl.publish(EventPublisherImpl.java:70)
          at sun.reflect.GeneratedMethodAccessor39.invoke(Unknown Source)
          at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
          at java.lang.reflect.Method.invoke(Unknown Source)
          at com.atlassian.multitenant.impl.MultiTenantComponentFactoryImpl$AbstractMultiTenantAwareInvocationHandler.invokeInternal(MultiTenantComponentFactoryImpl.java:181)
          at com.atlassian.multitenant.impl.MultiTenantComponentFactoryImpl$MultiTenantAwareInvocationHandler.invoke(MultiTenantComponentFactoryImpl.java:211)
          at $Proxy5.publish(Unknown Source)
          at com.atlassian.multitenant.event.DefaultPeeringEventPublisherManager.publish(DefaultPeeringEventPublisherManager.java:17)
          at com.atlassian.multitenant.event.PeeringEventPublisher.publish(PeeringEventPublisher.java:23)
          at com.atlassian.jira.event.issue.IssueEventDispatcher.dispatchEvent(IssueEventDispatcher.java:184)
          at com.atlassian.jira.event.issue.IssueEventDispatcher.dispatchEvent(IssueEventDispatcher.java:122)
          at com.atlassian.jira.issue.comments.DefaultCommentManager.dispatchEvent(DefaultCommentManager.java:352)
          at com.atlassian.jira.issue.comments.DefaultCommentManager.create(DefaultCommentManager.java:216)
          at com.atlassian.jira.issue.comments.DefaultCommentManager.create(DefaultCommentManager.java:166)
          at com.atlassian.jira.issue.comments.DefaultCommentManager.create(DefaultCommentManager.java:160)
          at com.atlassian.jira.bc.issue.comment.DefaultCommentService.create(DefaultCommentService.java:167)
          at com.atlassian.jira.bc.issue.comment.DefaultCommentService.create(DefaultCommentService.java:154)
          at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
          at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
          at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
          at java.lang.reflect.Method.invoke(Unknown Source)
          at com.atlassian.multitenant.impl.MultiTenantComponentFactoryImpl$AbstractMultiTenantAwareInvocationHandler.invokeInternal(MultiTenantComponentFactoryImpl.java:181)
          at com.atlassian.multitenant.impl.MultiTenantComponentFactoryImpl$MultiTenantAwareInvocationHandler.invoke(MultiTenantComponentFactoryImpl.java:211)
          at $Proxy195.create(Unknown Source)
          at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
          at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
          at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
          at java.lang.reflect.Method.invoke(Unknown Source)
          at com.atlassian.plugin.osgi.hostcomponents.impl.DefaultComponentRegistrar$ContextClassLoaderSettingInvocationHandler.invoke(DefaultComponentRegistrar.java:129)
          at $Proxy195.create(Unknown Source)
          at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
          at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
          at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
          at java.lang.reflect.Method.invoke(Unknown Source)
          at com.atlassian.plugin.osgi.bridge.external.HostComponentFactoryBean$DynamicServiceInvocationHandler.invoke(HostComponentFactoryBean.java:154)
          at $Proxy195.create(Unknown Source)
          at com.atlassian.jira.rest.v2.issue.CommentResource.addComment(CommentResource.java:266)
          at com.atlassian.jira.rest.v2.issue.IssueResource.addComment(IssueResource.java:1332)
          at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
          at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
          at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
          at java.lang.reflect.Method.invoke(Unknown Source)
          at com.atlassian.plugins.rest.common.interceptor.impl.DispatchProviderHelper$ResponseOutInvoker$1.invoke(DispatchProviderHelper.java:234)
          at com.atlassian.plugins.rest.common.interceptor.impl.DispatchProviderHelper$1.intercept(DispatchProviderHelper.java:100)
          at com.atlassian.plugins.rest.common.interceptor.impl.DefaultMethodInvocation.invoke(DefaultMethodInvocation.java:61)
          at com.atlassian.plugins.rest.common.expand.interceptor.ExpandInterceptor.intercept(ExpandInterceptor.java:38)
          at com.atlassian.plugins.rest.common.interceptor.impl.DefaultMethodInvocation.invoke(DefaultMethodInvocation.java:61)
          at com.atlassian.jira.rest.exception.ExceptionInterceptor.intercept(ExceptionInterceptor.java:59)
          at com.atlassian.plugins.rest.common.interceptor.impl.DefaultMethodInvocation.invoke(DefaultMethodInvocation.java:61)
          at com.atlassian.jira.rest.v2.issue.scope.RequestScopeInterceptor.intercept(RequestScopeInterceptor.java:43)
          at com.atlassian.plugins.rest.common.interceptor.impl.DefaultMethodInvocation.invoke(DefaultMethodInvocation.java:61)
          at com.atlassian.plugins.rest.common.interceptor.impl.DispatchProviderHelper.invokeMethodWithInterceptors(DispatchProviderHelper.java:132)
          at com.atlassian.plugins.rest.common.interceptor.impl.DispatchProviderHelper$ResponseOutInvoker._dispatch(DispatchProviderHelper.java:230)
          at com.sun.jersey.server.impl.model.method.dispatch.ResourceJavaMethodDispatcher.dispatch(ResourceJavaMethodDispatcher.java:75)
          at com.sun.jersey.server.impl.uri.rules.HttpMethodRule.accept(HttpMethodRule.java:288)
          at com.sun.jersey.server.impl.uri.rules.RightHandPathRule.accept(RightHandPathRule.java:147)
          at com.sun.jersey.server.impl.uri.rules.ResourceClassRule.accept(ResourceClassRule.java:108)
          at com.sun.jersey.server.impl.uri.rules.RightHandPathRule.accept(RightHandPathRule.java:147)
          at com.sun.jersey.server.impl.uri.rules.RootResourceClassesRule.accept(RootResourceClassesRule.java:84)
          at com.sun.jersey.server.impl.application.WebApplicationImpl._handleRequest(WebApplicationImpl.java:1469)
          at com.sun.jersey.server.impl.application.WebApplicationImpl._handleRequest(WebApplicationImpl.java:1400)
          at com.sun.jersey.server.impl.application.WebApplicationImpl.handleRequest(WebApplicationImpl.java:1349)
          at com.sun.jersey.server.impl.application.WebApplicationImpl.handleRequest(WebApplicationImpl.java:1339)
          at com.sun.jersey.spi.container.servlet.WebComponent.service(WebComponent.java:416)
          at com.sun.jersey.spi.container.servlet.ServletContainer.service(ServletContainer.java:537)
          at com.atlassian.plugins.rest.module.RestDelegatingServletFilter$JerseyOsgiServletContainer.doFilter(RestDelegatingServletFilter.java:178)
          at com.sun.jersey.spi.container.servlet.ServletContainer.doFilter(ServletContainer.java:795)
          at com.atlassian.plugins.rest.module.RestDelegatingServletFilter.doFilter(RestDelegatingServletFilter.java:73)
          at com.atlassian.plugin.servlet.filter.DelegatingPluginFilter.doFilter(DelegatingPluginFilter.java:74)
          at com.atlassian.plugin.servlet.filter.IteratingFilterChain.doFilter(IteratingFilterChain.java:42)
          at com.atlassian.plugin.servlet.filter.ServletFilterModuleContainerFilter.doFilter(ServletFilterModuleContainerFilter.java:77)
          at com.atlassian.plugin.servlet.filter.ServletFilterModuleContainerFilter.doFilter(ServletFilterModuleContainerFilter.java:63)
          at com.atlassian.plugin.servlet.filter.DelegatingPluginFilter.doFilter(DelegatingPluginFilter.java:74)
          at com.atlassian.plugin.servlet.filter.IteratingFilterChain.doFilter(IteratingFilterChain.java:42)
          at com.atlassian.plugin.servlet.filter.DelegatingPluginFilter$1.doFilter(DelegatingPluginFilter.java:66)
          at com.atlassian.plugins.rest.module.servlet.RestServletUtilsUpdaterFilter.doFilterInternal(RestServletUtilsUpdaterFilter.java:26)
          at com.atlassian.plugins.rest.module.servlet.RestServletUtilsUpdaterFilter.doFilter(RestServletUtilsUpdaterFilter.java:40)
          at com.atlassian.plugin.servlet.filter.DelegatingPluginFilter.doFilter(DelegatingPluginFilter.java:74)
          at com.atlassian.plugin.servlet.filter.IteratingFilterChain.doFilter(IteratingFilterChain.java:42)
          at com.atlassian.plugin.servlet.filter.DelegatingPluginFilter$1.doFilter(DelegatingPluginFilter.java:66)
          at com.atlassian.applinks.core.rest.context.ContextFilter.doFilter(ContextFilter.java:25)
          at com.atlassian.plugin.servlet.filter.DelegatingPluginFilter.doFilter(DelegatingPluginFilter.java:74)
          at com.atlassian.plugin.servlet.filter.IteratingFilterChain.doFilter(IteratingFilterChain.java:42)
          at com.atlassian.plugin.servlet.filter.ServletFilterModuleContainerFilter.doFilter(ServletFilterModuleContainerFilter.java:77)
          at com.atlassian.plugin.servlet.filter.ServletFilterModuleContainerFilter.doFilter(ServletFilterModuleContainerFilter.java:63)
          at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:235)
          at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
          at com.atlassian.jira.web.filters.accesslog.AccessLogFilter.executeRequest(AccessLogFilter.java:103)
          at com.atlassian.jira.web.filters.accesslog.AccessLogFilter.doFilter(AccessLogFilter.java:87)
          at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:235)
          at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
          at com.atlassian.jira.security.xsrf.XsrfTokenAdditionRequestFilter.doFilter(XsrfTokenAdditionRequestFilter.java:54)
          at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:235)
          at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
          at com.atlassian.plugin.servlet.filter.IteratingFilterChain.doFilter(IteratingFilterChain.java:46)
          at com.atlassian.plugin.servlet.filter.DelegatingPluginFilter$1.doFilter(DelegatingPluginFilter.java:66)
          at com.atlassian.labs.remoteapps.modules.permissions.ApiScopingFilter.doFilter(ApiScopingFilter.java:60)
          at com.atlassian.plugin.servlet.filter.DelegatingPluginFilter.doFilter(DelegatingPluginFilter.java:74)
          at com.atlassian.plugin.servlet.filter.IteratingFilterChain.doFilter(IteratingFilterChain.java:42)
          at com.atlassian.plugin.servlet.filter.ServletFilterModuleContainerFilter.doFilter(ServletFilterModuleContainerFilter.java:77)
          at com.atlassian.plugin.servlet.filter.ServletFilterModuleContainerFilter.doFilter(ServletFilterModuleContainerFilter.java:63)
          at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:235)
          at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
          at com.atlassian.seraph.filter.SecurityFilter.doFilter(SecurityFilter.java:234)
          at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:235)
          at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
          at com.atlassian.security.auth.trustedapps.filter.TrustedApplicationsFilter.doFilter(TrustedApplicationsFilter.java:98)
          at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:235)
          at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
          at com.atlassian.seraph.filter.BaseLoginFilter.doFilter(BaseLoginFilter.java:169)
          at com.atlassian.jira.web.filters.JiraLoginFilter.doFilter(JiraLoginFilter.java:70)
          at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:235)
          at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
          at com.atlassian.plugin.servlet.filter.IteratingFilterChain.doFilter(IteratingFilterChain.java:46)
          at com.atlassian.plugin.servlet.filter.DelegatingPluginFilter$1.doFilter(DelegatingPluginFilter.java:66)
          at com.atlassian.oauth.serviceprovider.internal.servlet.OAuthFilter.doFilter(OAuthFilter.java:71)
          at com.atlassian.plugin.servlet.filter.DelegatingPluginFilter.doFilter(DelegatingPluginFilter.java:74)
          at com.atlassian.plugin.servlet.filter.IteratingFilterChain.doFilter(IteratingFilterChain.java:42)
          at com.atlassian.plugin.servlet.filter.DelegatingPluginFilter$1.doFilter(DelegatingPluginFilter.java:66)
          at com.atlassian.plugins.rest.module.servlet.RestSeraphFilter.doFilter(RestSeraphFilter.java:40)
          at com.atlassian.plugin.servlet.filter.DelegatingPluginFilter.doFilter(DelegatingPluginFilter.java:74)
          at com.atlassian.plugin.servlet.filter.IteratingFilterChain.doFilter(IteratingFilterChain.java:42)
          at com.atlassian.plugin.servlet.filter.ServletFilterModuleContainerFilter.doFilter(ServletFilterModuleContainerFilter.java:77)
          at com.atlassian.plugin.servlet.filter.ServletFilterModuleContainerFilter.doFilter(ServletFilterModuleContainerFilter.java:63)
          at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:235)
          at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
          at com.atlassian.johnson.filters.AbstractJohnsonFilter.doFilter(AbstractJohnsonFilter.java:71)
          at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:235)
          at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
          at org.tuckey.web.filters.urlrewrite.UrlRewriteFilter.doFilter(UrlRewriteFilter.java:738)
          at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:235)
          at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
          at com.atlassian.gzipfilter.GzipFilter.doFilterInternal(GzipFilter.java:74)
          at com.atlassian.gzipfilter.GzipFilter.doFilter(GzipFilter.java:51)
          at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:235)
          at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
          at com.atlassian.plugin.servlet.filter.IteratingFilterChain.doFilter(IteratingFilterChain.java:46)
          at com.atlassian.plugin.servlet.filter.DelegatingPluginFilter$1.doFilter(DelegatingPluginFilter.java:66)
          at com.atlassian.labs.remoteapps.modules.oauth.OAuth2LOFilter.doFilter(OAuth2LOFilter.java:70)
          at com.atlassian.plugin.servlet.filter.DelegatingPluginFilter.doFilter(DelegatingPluginFilter.java:74)
          at com.atlassian.plugin.servlet.filter.IteratingFilterChain.doFilter(IteratingFilterChain.java:42)
          at com.atlassian.plugin.servlet.filter.DelegatingPluginFilter$1.doFilter(DelegatingPluginFilter.java:66)
          at com.atlassian.labs.remoteapps.util.http.bigpipe.RequestIdSettingFilter.doFilter(RequestIdSettingFilter.java:22)
          at com.atlassian.plugin.servlet.filter.DelegatingPluginFilter.doFilter(DelegatingPluginFilter.java:74)
          at com.atlassian.plugin.servlet.filter.IteratingFilterChain.doFilter(IteratingFilterChain.java:42)
          at com.atlassian.plugin.servlet.filter.DelegatingPluginFilter$1.doFilter(DelegatingPluginFilter.java:66)
          at com.sysbliss.jira.plugins.workflow.servlet.JWDSendRedirectFilter.doFilter(JWDSendRedirectFilter.java:25)
          at com.atlassian.plugin.servlet.filter.DelegatingPluginFilter.doFilter(DelegatingPluginFilter.java:74)
          at com.atlassian.plugin.servlet.filter.IteratingFilterChain.doFilter(IteratingFilterChain.java:42)
          at com.atlassian.plugin.servlet.filter.ServletFilterModuleContainerFilter.doFilter(ServletFilterModuleContainerFilter.java:77)
          at com.atlassian.plugin.servlet.filter.ServletFilterModuleContainerFilter.doFilter(ServletFilterModuleContainerFilter.java:63)
          at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:235)
          at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
          at com.atlassian.jira.web.filters.steps.ChainedFilterStepRunner.doFilter(ChainedFilterStepRunner.java:78)
          at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:235)
          at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
          at com.atlassian.core.filters.cache.AbstractCachingFilter.doFilter(AbstractCachingFilter.java:33)
          at com.atlassian.core.filters.AbstractHttpFilter.doFilter(AbstractHttpFilter.java:31)
          at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:235)
          at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
          at com.atlassian.core.filters.encoding.AbstractEncodingFilter.doFilter(AbstractEncodingFilter.java:41)
          at com.atlassian.core.filters.AbstractHttpFilter.doFilter(AbstractHttpFilter.java:31)
          at com.atlassian.jira.web.filters.PathMatchingEncodingFilter.doFilter(PathMatchingEncodingFilter.java:45)
          at com.atlassian.core.filters.AbstractHttpFilter.doFilter(AbstractHttpFilter.java:31)
          at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:235)
          at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
          at com.atlassian.jira.web.monitor.ActiveRequestsFilter$PassToChainFilterFunc.doFilter(ActiveRequestsFilter.java:346)
          at com.atlassian.jira.web.monitor.ActiveRequestsFilter$DebugLogFilterFunc.doFilter(ActiveRequestsFilter.java:463)
          at com.atlassian.jira.web.monitor.ActiveRequestsFilter.doFilter(ActiveRequestsFilter.java:173)
          at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:235)
          at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
          at com.atlassian.jira.startup.JiraStartupChecklistFilter.doFilter(JiraStartupChecklistFilter.java:75)
          at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:235)
          at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
          at com.atlassian.multitenant.servlet.MultiTenantServletFilter.doFilter(MultiTenantServletFilter.java:91)
          at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:235)
          at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
          at com.atlassian.jira.web.filters.steps.ChainedFilterStepRunner.doFilter(ChainedFilterStepRunner.java:78)
          at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:235)
          at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
          at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:233)
          at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:191)
          at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:127)
          at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:102)
          at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:109)
          at org.apache.catalina.valves.AccessLogValve.invoke(AccessLogValve.java:554)
          at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:298)
          at org.apache.coyote.http11.Http11Processor.process(Http11Processor.java:859)
          at org.apache.coyote.http11.Http11Protocol$Http11ConnectionHandler.process(Http11Protocol.java:588)
          at org.apache.tomcat.util.net.JIoEndpoint$Worker.run(JIoEndpoint.java:489)
          at java.lang.Thread.run(Unknown Source)
  Caused by: java.lang.NullPointerException
          at com.google.common.base.Preconditions.checkNotNull(Preconditions.java:187)
          at com.atlassian.jira.notification.NotificationRecipient.<init>(NotificationRecipient.java:36)
          at com.atlassian.jira.notification.type.UserToRecipient.apply(UserToRecipient.java:21)
          at com.atlassian.jira.notification.type.UserToRecipient.apply(UserToRecipient.java:14)
          at com.google.common.collect.Lists$TransformingRandomAccessList.get(Lists.java:451)
          at java.util.AbstractList$Itr.next(Unknown Source)
          at com.atlassian.jira.notification.DefaultNotificationSchemeManager.getRecipients(DefaultNotificationSchemeManager.java:373)
          at com.atlassian.jira.notification.DefaultNotificationSchemeManager.getRecipients(DefaultNotificationSchemeManager.java:408)
          at com.atlassian.jira.event.listeners.mention.MentionEventListener.onIssueEvent(MentionEventListener.java:42)
          at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
          at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
          at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
          at java.lang.reflect.Method.invoke(Unknown Source)
          at com.atlassian.event.internal.SingleParameterMethodListenerInvoker.invoke(SingleParameterMethodListenerInvoker.java:36)
          ... 195 more
  

Workaround

Delete the removed user from the Watcher list, via the database:
Backup your database first

select * from userassociation where source_name = 'testuser' and association_type = 'WatchIssue';