NullPointerException occurs when we try to create a diff graph with a graph in a different folder. We can reproduce with the following steps.
 # Open a graph file. Open a graph.
 # Select a graph in different folder in the Outline window.
 # Right-click and choose "Difference to current graph".
 # NullPointerException occurs.

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[INFO] SEVERE [global]
[INFO] java.lang.NullPointerException: Cannot invoke "com.sun.hotspot.igv.data.Folder.getElements()" because the return value of "com.sun.hotspot.igv.data.Group.getParent()" is null
[INFO] at com.sun.hotspot.igv.data.Group.getDisplayName(Group.java:129)
[INFO] at com.sun.hotspot.igv.view.EditorTopComponent.graphChanged(EditorTopComponent.java:252)
[INFO] at com.sun.hotspot.igv.view.EditorTopComponent.<init>(EditorTopComponent.java:247)
[INFO] at com.sun.hotspot.igv.view.GraphViewerImplementation.view(GraphViewerImplementation.java:66)
[INFO] at com.sun.hotspot.igv.view.GraphViewerImplementation.viewDifference(GraphViewerImplementation.java:44)
[INFO] at com.sun.hotspot.igv.coordinator.actions.DiffGraphCookie.openDiff(DiffGraphCookie.java:65)
[INFO] at com.sun.hotspot.igv.coordinator.actions.DiffGraphAction.performAction(DiffGraphAction.java:42)
[INFO] at org.openide.util.actions.NodeAction$DelegateAction$1.run(NodeAction.java:561)
[INFO] at org.openide.util.actions.ActionInvoker$1.run(ActionInvoker.java:70)
[INFO] at org.openide.util.actions.ActionInvoker.doPerformAction(ActionInvoker.java:91)
[INFO] at org.openide.util.actions.ActionInvoker.invokeAction(ActionInvoker.java:74)
[INFO] at org.openide.util.actions.NodeAction$DelegateAction.actionPerformed(NodeAction.java:558)
[INFO] at java.desktop/javax.swing.AbstractButton.fireActionPerformed(AbstractButton.java:1972)
[INFO] at java.desktop/javax.swing.AbstractButton$Handler.actionPerformed(AbstractButton.java:2313)
[INFO] at java.desktop/javax.swing.DefaultButtonModel.fireActionPerformed(DefaultButtonModel.java:405)
[INFO] at java.desktop/javax.swing.DefaultButtonModel.setPressed(DefaultButtonModel.java:262)
[INFO] at java.desktop/javax.swing.AbstractButton.doClick(AbstractButton.java:374)
[INFO] at java.desktop/javax.swing.plaf.basic.BasicMenuItemUI.doClick(BasicMenuItemUI.java:1028)
[INFO] at java.desktop/com.apple.laf.AquaMenuItemUI.doClick(AquaMenuItemUI.java:157)
[INFO] at java.desktop/javax.swing.plaf.basic.BasicMenuItemUI$Handler.mouseReleased(BasicMenuItemUI.java:1072)
[INFO] at java.desktop/java.awt.Component.processMouseEvent(Component.java:6626)
[INFO] at java.desktop/javax.swing.JComponent.processMouseEvent(JComponent.java:3389)
[INFO] at java.desktop/java.awt.Component.processEvent(Component.java:6391)
[INFO] at java.desktop/java.awt.Container.processEvent(Container.java:2266)
[INFO] at java.desktop/java.awt.Component.dispatchEventImpl(Component.java:5001)
[INFO] at java.desktop/java.awt.Container.dispatchEventImpl(Container.java:2324)
[INFO] at java.desktop/java.awt.Component.dispatchEvent(Component.java:4833)
[INFO] at java.desktop/java.awt.LightweightDispatcher.retargetMouseEvent(Container.java:4948)
[INFO] at java.desktop/java.awt.LightweightDispatcher.processMouseEvent(Container.java:4575)
[INFO] at java.desktop/java.awt.LightweightDispatcher.dispatchEvent(Container.java:4516)
[INFO] at java.desktop/java.awt.Container.dispatchEventImpl(Container.java:2310)
[INFO] at java.desktop/java.awt.Window.dispatchEventImpl(Window.java:2780)
[INFO] at java.desktop/java.awt.Component.dispatchEvent(Component.java:4833)
[INFO] at java.desktop/java.awt.EventQueue.dispatchEventImpl(EventQueue.java:773)
[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.base/java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(ProtectionDomain.java:97)
[INFO] at java.desktop/java.awt.EventQueue$5.run(EventQueue.java:746)
[INFO] at java.desktop/java.awt.EventQueue$5.run(EventQueue.java:744)
[INFO] at java.base/java.security.AccessController.doPrivileged(AccessController.java:399)
[INFO] at java.base/java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(ProtectionDomain.java:86)
[INFO] at java.desktop/java.awt.EventQueue.dispatchEvent(EventQueue.java:743)
[INFO] at org.netbeans.core.TimableEventQueue.dispatchEvent(TimableEventQueue.java:136)
[INFO] [catch] 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)