java.lang.IllegalArgumentException: Called DataObject.find on null
   at org.openide.loaders.DataObject.find(DataObject.java:517)
   at org.netbeans.modules.refactoring.spi.BackupFacility2$DefaultImpl.checkChecksum(BackupFacility2.java:245)
   at org.netbeans.modules.refactoring.spi.BackupFacility2$DefaultImpl.access$200(BackupFacility2.java:208)
   at org.netbeans.modules.refactoring.spi.BackupFacility2$DefaultHandle.checkChecksum(BackupFacility2.java:198)
   at org.netbeans.modules.refactoring.spi.RefactoringCommit.rollback(RefactoringCommit.java:199)
   at org.netbeans.modules.refactoring.api.RefactoringSession.undoRefactoring(RefactoringSession.java:200)
   at org.netbeans.modules.refactoring.spi.impl.UndoManager$SessionUndoItem.undo(UndoManager.java:385)
   at org.netbeans.modules.refactoring.spi.impl.UndoManager$1.run(UndoManager.java:194)
   at org.netbeans.modules.refactoring.spi.impl.UndoManager.undo(UndoManager.java:220)
   at org.netbeans.modules.refactoring.spi.impl.UndoableWrapper$UndoableEditDelegate.undo(UndoableWrapper.java:124)
   at org.openide.text.WrapUndoEdit.undo(WrapUndoEdit.java:93)
   at org.openide.awt.UndoRedo$Manager.undoTo(UndoRedo.java:230)
   at org.openide.awt.UndoRedo$Manager.undo(UndoRedo.java:200)
   at org.openide.text.UndoRedoManager.access$301(UndoRedoManager.java:98)
   at org.openide.text.UndoRedoManager$DocLockedRun.run(UndoRedoManager.java:645)
   at org.netbeans.editor.GuardedDocument.runAtomic(GuardedDocument.java:317)
   at org.openide.text.UndoRedoManager$DocLockedRun.<init>(UndoRedoManager.java:618)
   at org.openide.text.UndoRedoManager$DocLockedRun.<init>(UndoRedoManager.java:610)
   at org.openide.text.UndoRedoManager$DocLockedRun.<init>(UndoRedoManager.java:606)
   at org.openide.text.UndoRedoManager.undo(UndoRedoManager.java:434)
   at org.netbeans.core.multiview.MultiViewPeer$DelegateUndoRedo.undo(MultiViewPeer.java:813)
   at org.openide.actions.UndoAction.performAction(UndoAction.java:186)
   at org.openide.util.actions.CallableSystemAction$1.run(CallableSystemAction.java:129)
   at org.openide.util.actions.ActionInvoker$1.run(ActionInvoker.java:95)
   at org.openide.util.actions.ActionInvoker.doPerformAction(ActionInvoker.java:116)
   at org.openide.util.actions.ActionInvoker.invokeAction(ActionInvoker.java:99)
   at org.openide.util.actions.CallableSystemAction.actionPerformed(CallableSystemAction.java:126)
   at org.netbeans.modules.editor.NbEditorKit$NbUndoAction.actionPerformed(NbEditorKit.java:566)
   at org.netbeans.editor.BaseAction.actionPerformed(BaseAction.java:339)
   at javax.swing.SwingUtilities.notifyAction(SwingUtilities.java:1661)
   at javax.swing.JComponent.processKeyBinding(JComponent.java:2879)
   at javax.swing.JComponent.processKeyBindings(JComponent.java:2926)
   at javax.swing.JComponent.processKeyEvent(JComponent.java:2842)
   at java.awt.Component.processEvent(Component.java:6282)
   at java.awt.Container.processEvent(Container.java:2229)
   at java.awt.Component.dispatchEventImpl(Component.java:4861)
   at java.awt.Container.dispatchEventImpl(Container.java:2287)
   at java.awt.Component.dispatchEvent(Component.java:4687)
   at java.awt.KeyboardFocusManager.redispatchEvent(KeyboardFocusManager.java:1890)
   at java.awt.DefaultKeyboardFocusManager.dispatchKeyEvent(DefaultKeyboardFocusManager.java:752)
   at java.awt.DefaultKeyboardFocusManager.preDispatchKeyEvent(DefaultKeyboardFocusManager.java:1017)
   at java.awt.DefaultKeyboardFocusManager.typeAheadAssertions(DefaultKeyboardFocusManager.java:889)
   at java.awt.DefaultKeyboardFocusManager.dispatchEvent(DefaultKeyboardFocusManager.java:717)
   at java.awt.Component.dispatchEventImpl(Component.java:4731)
   at java.awt.Container.dispatchEventImpl(Container.java:2287)
   at java.awt.Window.dispatchEventImpl(Window.java:2713)
   at java.awt.Component.dispatchEvent(Component.java:4687)
   at java.awt.EventQueue.dispatchEventImpl(EventQueue.java:707)
   at java.awt.EventQueue.access$000(EventQueue.java:101)
   at java.awt.EventQueue$3.run(EventQueue.java:666)
   at java.awt.EventQueue$3.run(EventQueue.java:664)
   at java.security.AccessController.doPrivileged(AccessController.java:0)
   at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:76)
   at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:87)
   at java.awt.EventQueue$4.run(EventQueue.java:680)
   at java.awt.EventQueue$4.run(EventQueue.java:678)
   at java.security.AccessController.doPrivileged(AccessController.java:0)
   at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:76)
   at java.awt.EventQueue.dispatchEvent(EventQueue.java:677)
   at org.netbeans.core.TimableEventQueue.dispatchEvent(TimableEventQueue.java:158)
   at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:211)
   at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:128)
   at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:117)
   at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:113)
   at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:105)
   at java.awt.EventDispatchThread.run(EventDispatchThread.java:90)