postfunction "Add or remove watchers" doesn't work

I'm using this postfunction in the Create action of an issue to add all users in a project role as watchers. Unfortunately it doesn't work.

Log output: 2017-12-07 16:15:12,726 http-nio-8082-exec-24 ERROR dietmar 975x538x1 cjc9rf fe80:0:0:0:449e:bf6:41c9:e589%13 /secure/CreateIssueDetails.jspa [c.f.j.p.workflowToolbox.shared.ReflectionHelper] *** Object o: com.atlassian.jira.issue.watchers.DefaultWatcherManager@4dcbec0b; Method method: public abstract com.atlassian.jira.issue.Issue com.atlassian.jira.issue.watchers.WatcherManager.startWatching(com.atlassian.jira.user.ApplicationUser,com.atlassian.jira.issue.Issue); Object... arguments: [Ljava.lang.Object;@387eaf90; Cause: null java.lang.reflect.InvocationTargetException at sun.reflect.GeneratedMethodAccessor856.invoke(Unknown Source) at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) at java.lang.reflect.Method.invoke(Method.java:498) at com.fca.jira.plugins.workflowToolbox.shared.ReflectionHelper.invokeMethod(ReflectionHelper.java:151) at com.fca.jira.plugins.workflowToolbox.shared.JIRACompatibilityHelper.startOrStopWatching(JIRACompatibilityHelper.java:580) at com.fca.jira.plugins.workflowToolbox.shared.JIRACompatibilityHelper.startWatching(JIRACompatibilityHelper.java:545) at com.fca.jira.plugins.workflowToolbox.AddRemoveWatcherFunction.execute(AddRemoveWatcherFunction.java:113) at com.opensymphony.workflow.AbstractWorkflow.executeFunction(AbstractWorkflow.java:1014) at com.opensymphony.workflow.AbstractWorkflow.transitionWorkflow(AbstractWorkflow.java:1407) at com.opensymphony.workflow.AbstractWorkflow.initialize(AbstractWorkflow.java:606) at com.atlassian.jira.workflow.OSWorkflowManager.createIssue(OSWorkflowManager.java:742) at com.atlassian.jira.issue.managers.DefaultIssueManager.createIssue(DefaultIssueManager.java:580) at com.atlassian.jira.issue.managers.DefaultIssueManager.createIssue(DefaultIssueManager.java:494) at com.atlassian.jira.issue.managers.RequestCachingIssueManager.createIssue(RequestCachingIssueManager.java:192) at com.atlassian.jira.bc.issue.DefaultIssueService.create(DefaultIssueService.java:238) at com.atlassian.jira.web.action.issue.CreateIssueDetails.createIssue(CreateIssueDetails.java:99) at com.atlassian.jira.web.action.issue.CreateIssueDetails.doExecute(CreateIssueDetails.java:78) ... 1 filtered at com.atlassian.jira.action.JiraActionSupport.execute(JiraActionSupport.java:63) ... 7 filtered at javax.servlet.http.HttpServlet.service(HttpServlet.java:729) ... 52 filtered at com.atlassian.greenhopper.jira.filters.ClassicBoardRouter.doFilter(ClassicBoardRouter.java:62) ... 12 filtered at com.atlassian.web.servlet.plugin.request.RedirectInterceptingFilter.doFilter(RedirectInterceptingFilter.java:21) ... 58 filtered at com.atlassian.jira.security.JiraSecurityFilter.lambda$doFilter$0(JiraSecurityFilter.java:66) ... 1 filtered at com.atlassian.jira.security.JiraSecurityFilter.doFilter(JiraSecurityFilter.java:64) ... 36 filtered at com.atlassian.jira.servermetrics.CorrelationIdPopulatorFilter.doFilter(CorrelationIdPopulatorFilter.java:30) ... 10 filtered at com.atlassian.web.servlet.plugin.request.RedirectInterceptingFilter.doFilter(RedirectInterceptingFilter.java:21) ... 4 filtered at com.atlassian.web.servlet.plugin.LocationCleanerFilter.doFilter(LocationCleanerFilter.java:36) ... 26 filtered at com.atlassian.jira.servermetrics.MetricsCollectorFilter.doFilter(MetricsCollectorFilter.java:25) ... 23 filtered at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142) at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617) at org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61) at java.lang.Thread.run(Thread.java:745) Caused by: java.lang.NullPointerException at com.google.common.base.Preconditions.checkNotNull(Preconditions.java:210) at com.google.common.cache.LocalCache$LocalManualCache.invalidate(LocalCache.java:4764) at com.atlassian.jira.issue.watchers.DefaultWatcherManager.updateWatch(DefaultWatcherManager.java:172) at com.atlassian.jira.issue.watchers.DefaultWatcherManager.startWatching(DefaultWatcherManager.java:89) at com.atlassian.jira.issue.watchers.DefaultWatcherManager.startWatching(DefaultWatcherManager.java:80) ... 1 filtered at java.lang.reflect.Method.invoke(Method.java:498) at com.atlassian.plugin.util.ContextClassLoaderSettingInvocationHandler.invoke(ContextClassLoaderSettingInvocationHandler.java:26) at com.sun.proxy.$Proxy263.startWatching(Unknown Source) ... 2 filtered at java.lang.reflect.Method.invoke(Method.java:498) at com.atlassian.plugin.osgi.bridge.external.HostComponentFactoryBean$DynamicServiceInvocationHandler.invoke(HostComponentFactoryBean.java:136) at com.sun.proxy.$Proxy263.startWatching(Unknown Source) ... 261 more

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