NullPointerException removing subscriptions: quartz triggers missing

Some people have reported errors when deleting subscriptions:

java.lang.NullPointerException at
com.atlassian.scheduler.OfBizJobStore.removeTrigger(OfBizJobStore.java:296) at
org.quartz.core.QuartzScheduler.unscheduleJob(QuartzScheduler.java:587) at
org.quartz.impl.StdScheduler.unscheduleJob(StdScheduler.java:251) at
com.atlassian.jira.issue.subscription.DefaultSubscriptionManager.deleteSubscription(DefaultSubscriptionManager.java:123) at
com.atlassian.jira.web.action.filter.FilterSubscription.doDelete(FilterSubscription.java:125) at
sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) at
sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39) at
sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25) at
java.lang.reflect.Method.invoke(Method.java:585) at webwork.action.ActionSupport.invokeCommand(ActionSupport.java:415) at
webwork.action.ActionSupport.execute(ActionSupport.java:144) at
com.atlassian.jira.action.JiraActionSupport.execute(JiraActionSupport.java:46) at
webwork.dispatcher.GenericDispatcher.executeAction(GenericDispatcher.java:131) at
com.atlassian.jira.web.dispatcher.JiraServletDispatcher.service(JiraServletDispatcher.java:186) at
javax.servlet.http.HttpServlet.service(HttpServlet.java:853) at
org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:247) at
org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:193) at
com.opensymphony.module.sitemesh.filter.PageFilter.parsePage(PageFilter.java:142) at
com.opensymphony.module.sitemesh.filter.PageFilter.doFilter(PageFilter.java:58) at
com.atlassian.jira.web.filters.SitemeshExcludePathFilter.doFilter(SitemeshExcludePathFilter.java:36) at
org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:213) at
org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:193) at
com.atlassian.seraph.filter.SecurityFilter.doFilter(SecurityFilter.java:168) at
org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:213) at
org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:193) at
com.atlassian.seraph.filter.LoginFilter.doFilter(LoginFilter.java:181) at
org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:213) at
org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:193) at
com.atlassian.util.profiling.filters.ProfilingFilter.doFilter(ProfilingFilter.java:132) at
com.atlassian.jira.web.filters.ProfilingAndErrorFilter.doFilter(ProfilingAndErrorFilter.java:25) at
org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:213) at
org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:193) at
com.atlassian.jira.web.filters.ActionCleanupDelayFilter.doFilter(ActionCleanupDelayFilter.java:37) at
org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:213) at
org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:193) at
com.atlassian.johnson.filters.JohnsonFilter.doFilter(JohnsonFilter.java:91) at
org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:213) at
org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:193) at
com.atlassian.jira.web.filters.gzip.GzipFilter.doFilter(GzipFilter.java:64) at
org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:213) at