java.util.ConcurrentModificationException
   at java.util.ArrayList$Itr.checkForComodification(ArrayList.java:819)
   at java.util.ArrayList$Itr.next(ArrayList.java:791)
   at org.netbeans.modules.debugger.jpda.visual.VisualDebuggerListener.engineRemoved(VisualDebuggerListener.java:354)
   at org.netbeans.api.debugger.DebuggerManager.fireEngineRemoved(DebuggerManager.java:1466)
   at org.netbeans.api.debugger.DebuggerManager.removeEngine(DebuggerManager.java:1650)
   at org.netbeans.api.debugger.Session.removeEngine(Session.java:447)
   at org.netbeans.api.debugger.DebuggerEngine$Destructor.killEngine(DebuggerEngine.java:241)
   at org.netbeans.modules.debugger.jpda.JPDADebuggerImpl.finish(JPDADebuggerImpl.java:1348)
   at org.netbeans.modules.debugger.jpda.actions.KillActionProvider.doAction(KillActionProvider.java:79)
   at org.netbeans.spi.debugger.ActionsProvider$1.run(ActionsProvider.java:130)
   at org.openide.util.RequestProcessor$Task.run(RequestProcessor.java:1454)
   at org.openide.util.RequestProcessor$Processor.run(RequestProcessor.java:2043)