Workflow Import Fails Due to Violation of Comparator Contract

Steps to reproduce

1. import the workflow xml file.
2. edit/view the workflow.
3. change the mode to Diagram View.
4. the following error will appears:

    "The JIRA server was contacted but has returned an error response. We are unsure of the result of this operation." 

5. with this stack trace in the atlassian-jira.log:

   2014-05-14 23:57:14,286 http-bio-8625-exec-23 ERROR admin 1437x240x1 1jvzekl 0:0:0:0:0:0:0:1 /rest/workflowDesigner/latest/workflows [common.error.jersey.ThrowableExceptionMapper] Uncaught exception thrown by REST service: java.lang.IllegalArgumentException: Comparison method violates its general contract!
   java.lang.RuntimeException: java.lang.IllegalArgumentException: Comparison method violates its general contract!
   	at com.sysbliss.jira.plugins.workflow.manager.WorkflowLayoutManagerImpl.getOldLayout(WorkflowLayoutManagerImpl.java:350)
   	at com.sysbliss.jira.plugins.workflow.manager.WorkflowLayoutManagerImpl.generateNewLayout(WorkflowLayoutManagerImpl.java:292)
   	at com.sysbliss.jira.plugins.workflow.manager.WorkflowLayoutManagerImpl.getNewLayoutJson(WorkflowLayoutManagerImpl.java:282)
   	at com.atlassian.jira.plugins.workflowdesigner.layout.WorkflowLayoutsImpl.getLayoutJson(WorkflowLayoutsImpl.java:95)
   	at com.atlassian.jira.plugins.workflowdesigner.rest.WorkflowsResource.getWorkflow(WorkflowsResource.java:67)  <+3>
   	at java.lang.reflect.Method.invoke(Unknown Source)  <+18> (DispatchProviderHelper.java:234) (DispatchProviderHelper.java:100) (DefaultMethodInvocation.java:61) (ExpandInterceptor.java:38) (DefaultMethodInvocation.java:61) (DispatchProviderHelper.java:132) (DispatchProviderHelper.java:230) (ResourceJavaMethodDispatcher.java:75) (HttpMethodRule.java:288) (ResourceClassRule.java:108) (RightHandPathRule.java:147) (RootResourceClassesRule.java:84) (WebApplicationImpl.java:1469) (WebApplicationImpl.java:1400) (WebApplicationImpl.java:1349) (WebApplicationImpl.java:1339) (WebComponent.java:416) (ServletContainer.java:537)
   	at com.atlassian.plugins.rest.module.RestDelegatingServletFilter$JerseyOsgiServletContainer.doFilter(RestDelegatingServletFilter.java:178)  <+1> (ServletContainer.java:795)
   	at com.atlassian.plugins.rest.module.RestDelegatingServletFilter.doFilter(RestDelegatingServletFilter.java:73)  <+16> (DelegatingPluginFilter.java:74) (IteratingFilterChain.java:42) (ServletFilterModuleContainerFilter.java:77) (ServletFilterModuleContainerFilter.java:63) (DelegatingPluginFilter.java:74) (IteratingFilterChain.java:42) (DelegatingPluginFilter.java:66) (RestServletUtilsUpdaterFilter.java:26) (RestServletUtilsUpdaterFilter.java:40) (DelegatingPluginFilter.java:74) (IteratingFilterChain.java:42) (DelegatingPluginFilter.java:66) (ContextFilter.java:25) (DelegatingPluginFilter.java:74) (IteratingFilterChain.java:42) (DelegatingPluginFilter.java:66)
   	at com.atlassian.mywork.client.filter.ServingRequestsFilter.doFilter(ServingRequestsFilter.java:37)  <+3> (DelegatingPluginFilter.java:74) (IteratingFilterChain.java:42) (DelegatingPluginFilter.java:66)
   	at com.atlassian.plugins.cors.CorsFilter.doFilter(CorsFilter.java:65)  <+3> (DelegatingPluginFilter.java:74) (IteratingFilterChain.java:42) (DelegatingPluginFilter.java:66)
   	at com.atlassian.prettyurls.filter.PrettyUrlsDispatcherFilter.doFilter(PrettyUrlsDispatcherFilter.java:60)  <+3> (DelegatingPluginFilter.java:74) (IteratingFilterChain.java:42) (DelegatingPluginFilter.java:66)
   	at com.atlassian.prettyurls.filter.PrettyUrlsSiteMeshFilter.doFilter(PrettyUrlsSiteMeshFilter.java:92)  <+3> (DelegatingPluginFilter.java:74) (IteratingFilterChain.java:42) (DelegatingPluginFilter.java:66)
   	at com.atlassian.prettyurls.filter.PrettyUrlsMatcherFilter.doFilter(PrettyUrlsMatcherFilter.java:56)  <+3> (DelegatingPluginFilter.java:74) (IteratingFilterChain.java:42) (DelegatingPluginFilter.java:66)
   	at com.atlassian.labs.botkiller.BotKillerFilter.doFilter(BotKillerFilter.java:36)  <+18> (DelegatingPluginFilter.java:74) (IteratingFilterChain.java:42) (ServletFilterModuleContainerFilter.java:77) (ServletFilterModuleContainerFilter.java:63) (ApplicationFilterChain.java:243) (ApplicationFilterChain.java:210) (AccessLogFilter.java:103) (AccessLogFilter.java:87) (ApplicationFilterChain.java:243) (ApplicationFilterChain.java:210) (XsrfTokenAdditionRequestFilter.java:54) (ApplicationFilterChain.java:243) (ApplicationFilterChain.java:210) (ChainedFilterStepRunner.java:87) (ApplicationFilterChain.java:243) (ApplicationFilterChain.java:210) (IteratingFilterChain.java:46) (DelegatingPluginFilter.java:66)
   	at com.atlassian.prettyurls.filter.PrettyUrlsCombinedMatchDispatcherFilter.doFilter(PrettyUrlsCombinedMatchDispatcherFilter.java:61)  <+22> (DelegatingPluginFilter.java:74) (IteratingFilterChain.java:42) (ServletFilterModuleContainerFilter.java:77) (ServletFilterModuleContainerFilter.java:63) (ApplicationFilterChain.java:243) (ApplicationFilterChain.java:210) (SecurityFilter.java:234) (ApplicationFilterChain.java:243) (ApplicationFilterChain.java:210) (TrustedApplicationsFilter.java:100) (ApplicationFilterChain.java:243) (ApplicationFilterChain.java:210) (BaseLoginFilter.java:169) (JiraLoginFilter.java:70) (ApplicationFilterChain.java:243) (ApplicationFilterChain.java:210) (IteratingFilterChain.java:46) (DelegatingPluginFilter.java:66) (OAuthFilter.java:61) (DelegatingPluginFilter.java:74) (IteratingFilterChain.java:42) (DelegatingPluginFilter.java:66)
   	at com.atlassian.plugins.rest.module.servlet.RestSeraphFilter.doFilter(RestSeraphFilter.java:40)  <+3> (DelegatingPluginFilter.java:74) (IteratingFilterChain.java:42) (DelegatingPluginFilter.java:66)
   	at com.atlassian.prettyurls.filter.PrettyUrlsCombinedMatchDispatcherFilter.doFilter(PrettyUrlsCombinedMatchDispatcherFilter.java:61)  <+9> (DelegatingPluginFilter.java:74) (IteratingFilterChain.java:42) (ServletFilterModuleContainerFilter.java:77) (ServletFilterModuleContainerFilter.java:63) (ApplicationFilterChain.java:243) (ApplicationFilterChain.java:210) (AbstractJohnsonFilter.java:71) (ApplicationFilterChain.java:243) (ApplicationFilterChain.java:210)
   	at org.tuckey.web.filters.urlrewrite.RuleChain.handleRewrite(RuleChain.java:176)
   	at org.tuckey.web.filters.urlrewrite.RuleChain.doRules(RuleChain.java:145)
   	at org.tuckey.web.filters.urlrewrite.UrlRewriter.processRequest(UrlRewriter.java:92)  <+10> (UrlRewriteFilter.java:394) (ApplicationFilterChain.java:243) (ApplicationFilterChain.java:210) (GzipFilter.java:74) (GzipFilter.java:51) (JiraGzipFilter.java:55) (ApplicationFilterChain.java:243) (ApplicationFilterChain.java:210) (IteratingFilterChain.java:46) (DelegatingPluginFilter.java:66)
   	at com.atlassian.prettyurls.filter.PrettyUrlsCombinedMatchDispatcherFilter.doFilter(PrettyUrlsCombinedMatchDispatcherFilter.java:61)  <+40> (DelegatingPluginFilter.java:74) (IteratingFilterChain.java:42) (ServletFilterModuleContainerFilter.java:77) (ServletFilterModuleContainerFilter.java:63) (ApplicationFilterChain.java:243) (ApplicationFilterChain.java:210) (ChainedFilterStepRunner.java:87) (ApplicationFilterChain.java:243) (ApplicationFilterChain.java:210) (AbstractCachingFilter.java:33) (AbstractHttpFilter.java:31) (ApplicationFilterChain.java:243) (ApplicationFilterChain.java:210) (AbstractEncodingFilter.java:41) (AbstractHttpFilter.java:31) (PathMatchingEncodingFilter.java:45) (AbstractHttpFilter.java:31) (ApplicationFilterChain.java:243) (ApplicationFilterChain.java:210) (JiraStartupChecklistFilter.java:78) (ApplicationFilterChain.java:243) (ApplicationFilterChain.java:210) (MultipartBoundaryCheckFilter.java:41) (ApplicationFilterChain.java:243) (ApplicationFilterChain.java:210) (ChainedFilterStepRunner.java:87) (JiraFirstFilter.java:57) (ApplicationFilterChain.java:243) (ApplicationFilterChain.java:210) (StandardWrapperValve.java:222) (StandardContextValve.java:123) (AuthenticatorBase.java:502) (StandardHostValve.java:171) (ErrorReportValve.java:100) (StandardEngineValve.java:118) (AccessLogValve.java:953) (CoyoteAdapter.java:408) (AbstractHttp11Processor.java:1041) (AbstractProtocol.java:603) (JIoEndpoint.java:312)
   	at java.util.concurrent.ThreadPoolExecutor.runWorker(Unknown Source)
   	at java.util.concurrent.ThreadPoolExecutor$Worker.run(Unknown Source)
   	at java.lang.Thread.run(Unknown Source)
   Caused by: java.lang.IllegalArgumentException: Comparison method violates its general contract!
   	at java.util.ComparableTimSort.mergeLo(Unknown Source)
   	at java.util.ComparableTimSort.mergeAt(Unknown Source)
   	at java.util.ComparableTimSort.mergeForceCollapse(Unknown Source)
   	at java.util.ComparableTimSort.sort(Unknown Source)
   	at java.util.ComparableTimSort.sort(Unknown Source)
   	at com.mxgraph.layout.hierarchical.stage.mxCoordinateAssignment.rankMedianPosition(Unknown Source)
   	at com.mxgraph.layout.hierarchical.stage.mxCoordinateAssignment.medianPos(Unknown Source)
   	at com.mxgraph.layout.hierarchical.stage.mxCoordinateAssignment.execute(Unknown Source)
   	at com.mxgraph.layout.hierarchical.mxHierarchicalLayout.placementStage(Unknown Source)
   	at com.mxgraph.layout.hierarchical.mxHierarchicalLayout.run(Unknown Source)
   	at com.mxgraph.layout.hierarchical.mxHierarchicalLayout.execute(Unknown Source)
   	at com.sysbliss.jira.plugins.workflow.jgraph.JGraphxJWDLayoutProcessor.actuallyCalculateLayout(JGraphxJWDLayoutProcessor.java:98)
   	at com.sysbliss.jira.plugins.workflow.jgraph.JGraphxJWDLayoutProcessor.calculateLayout(JGraphxJWDLayoutProcessor.java:71)
   	at com.sysbliss.jira.plugins.workflow.manager.WorkflowLayoutManagerImpl.getLayoutForWorkflow(WorkflowLayoutManagerImpl.java:162)
   	at com.sysbliss.jira.plugins.workflow.manager.WorkflowLayoutManagerImpl.getOldLayout(WorkflowLayoutManagerImpl.java:346)