1) open a graph in group A
2) right click on another graph in group B
3) select "Difference to current graph"
4) get an java.lang.IllegalArgumentException

[INFO] SEVERE [org.openide.util.Exceptions]
[INFO] java.lang.IllegalArgumentException: No element of a node selection may be null.
[INFO] at org.openide.explorer.ExplorerManager$1AtomicSetSelectedNodes.checkAndSet(ExplorerManager.java:223)
[INFO] at org.openide.explorer.ExplorerManager$1AtomicSetSelectedNodes.run(ExplorerManager.java:274)
[INFO] at org.netbeans.modules.openide.util.DefaultMutexImplementation.readAccess(DefaultMutexImplementation.java:172)
[INFO] at org.netbeans.modules.openide.util.DefaultMutexImplementation$1R.run(DefaultMutexImplementation.java:1016)
[INFO] at org.openide.nodes.Children$ProjectManagerDeadlockDetector.execute(Children.java:1914)
[INFO] at org.netbeans.modules.openide.util.DefaultMutexImplementation.doWrapperAccess(DefaultMutexImplementation.java:1035)
[INFO] at org.netbeans.modules.openide.util.DefaultMutexImplementation.readAccess(DefaultMutexImplementation.java:162)
[INFO] at org.openide.util.Mutex.readAccess(Mutex.java:241)
[INFO] at org.openide.explorer.ExplorerManager.setSelectedNodes(ExplorerManager.java:279)
[INFO] [catch] at com.sun.hotspot.igv.coordinator.OutlineTopComponent.lambda$changed$2(OutlineTopComponent.java:248)
[INFO] at java.desktop/java.awt.event.InvocationEvent.dispatch(InvocationEvent.java:318)
[INFO] at java.desktop/java.awt.EventQueue.dispatchEventImpl(EventQueue.java:771)
[INFO] at java.desktop/java.awt.EventQueue$4.run(EventQueue.java:722)
[INFO] at java.desktop/java.awt.EventQueue$4.run(EventQueue.java:716)
[INFO] at java.base/java.security.AccessController.doPrivileged(AccessController.java:399)
[INFO] at java.base/java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(ProtectionDomain.java:86)
[INFO] at java.desktop/java.awt.EventQueue.dispatchEvent(EventQueue.java:741)
[INFO] at org.netbeans.core.TimableEventQueue.dispatchEvent(TimableEventQueue.java:136)
[INFO] at java.desktop/java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:203)
[INFO] at java.desktop/java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:124)
[INFO] at java.desktop/java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:113)
[INFO] at java.desktop/java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:109)
[INFO] at java.desktop/java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:101)
[INFO] at java.desktop/java.awt.EventDispatchThread.run(EventDispatchThread.java:90)