java.lang.AssertionError: Null children for accurate span
   at org.netbeans.modules.editor.lib2.view.ViewBuilder.replaceRepaintViews(ViewBuilder.java:1242)
   at org.netbeans.modules.editor.lib2.view.ViewBuilder.createReplaceRepaintViews(ViewBuilder.java:687)
   at org.netbeans.modules.editor.lib2.view.ViewUpdates.updateViewsByModification(ViewUpdates.java:311)
   at org.netbeans.modules.editor.lib2.view.ViewUpdates.insertUpdate(ViewUpdates.java:247)
   at sun.reflect.GeneratedMethodAccessor70.invoke(GeneratedMethodAccessor70.java:0)
   at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
   at java.lang.reflect.Method.invoke(Method.java:601)
   at org.openide.util.WeakListenerImpl$ProxyListener.invoke(WeakListenerImpl.java:457)
   at $Proxy22.insertUpdate(.java:0)
   at org.netbeans.lib.editor.util.swing.PriorityDocumentListenerList.insertUpdate(PriorityDocumentListenerList.java:85)
   at javax.swing.text.AbstractDocument.fireInsertUpdate(AbstractDocument.java:202)
   at org.netbeans.editor.BaseDocument.fireInsertUpdate(BaseDocument.java:1586)
   at org.netbeans.editor.BaseDocumentEvent.undo(BaseDocumentEvent.java:286)
   at org.netbeans.editor.GuardedDocumentEvent.undo(GuardedDocumentEvent.java:72)
   at org.netbeans.modules.editor.lib2.document.StableCompoundEdit.undo(StableCompoundEdit.java:93)
   at org.netbeans.editor.BaseDocument$AtomicCompoundEdit.undo(BaseDocument.java:2116)
   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:650)
   at org.netbeans.editor.GuardedDocument.runAtomic(GuardedDocument.java:317)
   at org.openide.text.UndoRedoManager$DocLockedRun.<init>(UndoRedoManager.java:623)
   at org.openide.text.UndoRedoManager$DocLockedRun.<init>(UndoRedoManager.java:615)
   at org.openide.text.UndoRedoManager$DocLockedRun.<init>(UndoRedoManager.java:611)
   at org.openide.text.UndoRedoManager.undo(UndoRedoManager.java:434)
   at org.netbeans.core.multiview.MultiViewPeer$DelegateUndoRedo.undo(MultiViewPeer.java:822)
   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:1664)
   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:1895)
   at java.awt.DefaultKeyboardFocusManager.dispatchKeyEvent(DefaultKeyboardFocusManager.java:762)
   at java.awt.DefaultKeyboardFocusManager.preDispatchKeyEvent(DefaultKeyboardFocusManager.java:1027)
   at java.awt.DefaultKeyboardFocusManager.typeAheadAssertions(DefaultKeyboardFocusManager.java:899)
   at java.awt.DefaultKeyboardFocusManager.dispatchEvent(DefaultKeyboardFocusManager.java:727)
   at java.awt.Component.dispatchEventImpl(Component.java:4731)
   at java.awt.Container.dispatchEventImpl(Container.java:2287)
   at java.awt.Window.dispatchEventImpl(Window.java:2719)
   at java.awt.Component.dispatchEvent(Component.java:4687)
   at java.awt.EventQueue.dispatchEventImpl(EventQueue.java:723)
   at java.awt.EventQueue.access$200(EventQueue.java:103)
   at java.awt.EventQueue$3.run(EventQueue.java:682)
   at java.awt.EventQueue$3.run(EventQueue.java:680)
   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:696)
   at java.awt.EventQueue$4.run(EventQueue.java:694)
   at java.security.AccessController.doPrivileged(AccessController.java:0)
   at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:76)
   at java.awt.EventQueue.dispatchEvent(EventQueue.java:693)
   at org.netbeans.core.TimableEventQueue.dispatchEvent(TimableEventQueue.java:158)
   at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:242)
   at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:161)
   at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:150)
   at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:146)
   at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:138)
   at java.awt.EventDispatchThread.run(EventDispatchThread.java:91)