java.lang.NullPointerException
        at org.netbeans.modules.editor.hints.AnnotationHolder.removeUpdate(AnnotationHolder.java:410)
        at org.netbeans.lib.editor.util.swing.PriorityDocumentListenerList.removeUpdate(PriorityDocumentListenerList.java:99)
        at javax.swing.text.AbstractDocument.fireRemoveUpdate(AbstractDocument.java:243)
        at org.netbeans.editor.BaseDocument.fireRemoveUpdate(BaseDocument.java:1626)
        at org.netbeans.editor.BaseDocumentEvent.undo(BaseDocumentEvent.java:312)
        at org.netbeans.editor.GuardedDocumentEvent.undo(GuardedDocumentEvent.java:69)
        at javax.swing.undo.CompoundEdit.undo(CompoundEdit.java:46)
        at org.netbeans.editor.BaseDocument$AtomicCompoundEdit.undo(BaseDocument.java:2181)
        at org.netbeans.editor.BaseDocument.breakAtomicLock(BaseDocument.java:1814)
        at org.netbeans.editor.GuardedDocument.runAtomicAsUser(GuardedDocument.java:360)
        at org.netbeans.editor.BaseKit$DefaultKeyTypedAction.actionPerformed(BaseKit.java:1061)
        at org.netbeans.editor.ext.ExtKit$ExtDefaultKeyTypedAction.actionPerformed(ExtKit.java:1043)
        at org.netbeans.editor.BaseAction.actionPerformed(BaseAction.java:328)
        at javax.swing.SwingUtilities.notifyAction(SwingUtilities.java:1636)
        at javax.swing.JComponent.processKeyBinding(JComponent.java:2851)
        at javax.swing.JComponent.processKeyBindings(JComponent.java:2886)
        at javax.swing.JComponent.processKeyEvent(JComponent.java:2814)
        at java.awt.Component.processEvent(Component.java:6040)
        at java.awt.Container.processEvent(Container.java:2041)
        at java.awt.Component.dispatchEventImpl(Component.java:4630)
        at java.awt.Container.dispatchEventImpl(Container.java:2099)
        at java.awt.Component.dispatchEvent(Component.java:4460)
        at java.awt.KeyboardFocusManager.redispatchEvent(KeyboardFocusManager.java:1848)
        at java.awt.DefaultKeyboardFocusManager.dispatchKeyEvent(DefaultKeyboardFocusManager.java:704)
        at java.awt.DefaultKeyboardFocusManager.preDispatchKeyEvent(DefaultKeyboardFocusManager.java:969)
        at java.awt.DefaultKeyboardFocusManager.typeAheadAssertions(DefaultKeyboardFocusManager.java:841)
        at java.awt.DefaultKeyboardFocusManager.dispatchEvent(DefaultKeyboardFocusManager.java:668)
        at java.awt.Component.dispatchEventImpl(Component.java:4502)
        at java.awt.Container.dispatchEventImpl(Container.java:2099)
        at java.awt.Window.dispatchEventImpl(Window.java:2475)
        at java.awt.Component.dispatchEvent(Component.java:4460)
        at java.awt.EventQueue.dispatchEvent(EventQueue.java:599)
        at org.netbeans.core.TimableEventQueue.dispatchEvent(TimableEventQueue.java:117)
        at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:269)
        at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:184)
        at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:174)
        at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:169)
        at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:161)
        at java.awt.EventDispatchThread.run(EventDispatchThread.java:122)