java.lang.NullPointerException
        at javax.swing.plaf.basic.BasicTreeUI$Handler.handleSelectionImpl(BasicTreeUI.java:3492)
        at javax.swing.plaf.basic.BasicTreeUI$Handler.handleSelection(BasicTreeUI.java:3484)
        at javax.swing.plaf.basic.BasicTreeUI$Handler.mousePressed(BasicTreeUI.java:3465)
        at java.awt.AWTEventMulticaster.mousePressed(AWTEventMulticaster.java:222)
        at java.awt.AWTEventMulticaster.mousePressed(AWTEventMulticaster.java:221)
        at java.awt.AWTEventMulticaster.mousePressed(AWTEventMulticaster.java:221)
        at java.awt.AWTEventMulticaster.mousePressed(AWTEventMulticaster.java:221)
        at java.awt.AWTEventMulticaster.mousePressed(AWTEventMulticaster.java:221)
        at java.awt.AWTEventMulticaster.mousePressed(AWTEventMulticaster.java:221)
        at java.awt.Component.processMouseEvent(Component.java:5580)
        at javax.swing.JComponent.processMouseEvent(JComponent.java:3135)
        at java.awt.Component.processEvent(Component.java:5348)
        at java.awt.Container.processEvent(Container.java:2010)
        at java.awt.Component.dispatchEventImpl(Component.java:4050)
        at java.awt.Container.dispatchEventImpl(Container.java:2068)
        at java.awt.Component.dispatchEvent(Component.java:3885)
        at java.awt.LightweightDispatcher.retargetMouseEvent(LightweightDispatcher.java:4256)
        at java.awt.LightweightDispatcher.processMouseEvent(LightweightDispatcher.java:3933)
        at java.awt.LightweightDispatcher.dispatchEvent(LightweightDispatcher.java:3866)
        at java.awt.Container.dispatchEventImpl(Container.java:2054)
        at java.awt.Window.dispatchEventImpl(Window.java:1791)
        at java.awt.Component.dispatchEvent(Component.java:3885)
        at java.awt.EventQueue.dispatchEvent(EventQueue.java:463)
        at org.netbeans.core.TimableEventQueue.dispatchEvent(TimableEventQueue.java:104)
        at java.awt.EventDispatchThread.pumpOneEventForHierarchy(EventDispatchThread.java:269)
        at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:190)
        at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:184)
        at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:176)
        at java.awt.EventDispatchThread.run(EventDispatchThread.java:110)