java.lang.NullPointerException
   at com.sun.java.swing.plaf.windows.WindowsTreeUI.ensureRowsAreVisible(WindowsTreeUI.java:74)
   at javax.swing.plaf.basic.BasicTreeUI.toggleExpandState(BasicTreeUI.java:2213)
   at javax.swing.plaf.basic.BasicTreeUI.handleExpandControlClick(BasicTreeUI.java:2196)
   at javax.swing.plaf.basic.BasicTreeUI.checkForClickInExpandControl(BasicTreeUI.java:2154)
   at javax.swing.plaf.basic.BasicTreeUI$Handler.handleSelection(BasicTreeUI.java:3516)
   at javax.swing.plaf.basic.BasicTreeUI$Handler.mousePressedDND(BasicTreeUI.java:3502)
   at javax.swing.plaf.basic.BasicTreeUI$Handler.mousePressed(BasicTreeUI.java:3461)
   at java.awt.AWTEventMulticaster.mousePressed(AWTEventMulticaster.java:263)
   at java.awt.AWTEventMulticaster.mousePressed(AWTEventMulticaster.java:262)
   at java.awt.Component.processMouseEvent(Component.java:6213)
   at javax.swing.JComponent.processMouseEvent(JComponent.java:3265)
   at java.awt.Component.processEvent(Component.java:5981)
   at java.awt.Container.processEvent(Container.java:2041)
   at java.awt.Component.dispatchEventImpl(Component.java:4583)
   at java.awt.Container.dispatchEventImpl(Container.java:2099)
   at java.awt.Component.dispatchEvent(Component.java:4413)
   at java.awt.LightweightDispatcher.retargetMouseEvent(LightweightDispatcher.java:4556)
   at java.awt.LightweightDispatcher.processMouseEvent(LightweightDispatcher.java:4217)
   at java.awt.LightweightDispatcher.dispatchEvent(LightweightDispatcher.java:4150)
   at java.awt.Container.dispatchEventImpl(Container.java:2085)
   at java.awt.Window.dispatchEventImpl(Window.java:2475)
   at java.awt.Component.dispatchEvent(Component.java:4413)
   at java.awt.EventQueue.dispatchEvent(EventQueue.java:599)
   at org.netbeans.core.TimableEventQueue.dispatchEvent(TimableEventQueue.java:162)
   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)