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This exception has no message.
java.lang.NullPointerException: 	at jd_back$1.valueChanged(jd_back.java:56)	at javax.swing.JTree.fireValueChanged(JTree.java:2406)	at javax.swing.JTree$TreeSelectionRedirector.valueChanged(JTree.java:2777)	at javax.swing.tree.DefaultTreeSelectionModel.fireValueChanged(DefaultTreeSelectionModel.java:629)	at javax.swing.tree.DefaultTreeSelectionModel.clearSelection(DefaultTreeSelectionModel.java:570)	at javax.swing.tree.VariableHeightLayoutCache.rebuild(VariableHeightLayoutCache.java:740)	at javax.swing.tree.VariableHeightLayoutCache.treeStructureChanged(VariableHeightLayoutCache.java:626)	at javax.swing.plaf.basic.BasicTreeUI$TreeModelHandler.treeStructureChanged(BasicTreeUI.java:2469)	at javax.swing.tree.DefaultTreeModel.fireTreeStructureChanged(DefaultTreeModel.java:561)	at javax.swing.tree.DefaultTreeModel.reload(DefaultTreeModel.java:277)	at javax.swing.tree.DefaultTreeModel.reload(DefaultTreeModel.java:194)	at jd_back.updateTree(jd_back.java:109)	at jd_back$command_listner.mouseClicked(jd_back.java:73)	at java.awt.AWTEventMulticaster.mouseClicked(AWTEventMulticaster.java:208)	at java.awt.Component.processMouseEvent(Component.java:5137)	at java.awt.Component.processEvent(Component.java:4931)	at java.awt.Container.processEvent(Container.java:1566)	at java.awt.Component.dispatchEventImpl(Component.java:3639)	at java.awt.Container.dispatchEventImpl(Container.java:1623)	at java.awt.Component.dispatchEvent(Component.java:3480)	at java.awt.LightweightDispatcher.retargetMouseEvent(Container.java:3450)	at java.awt.LightweightDispatcher.processMouseEvent(Container.java:3174)	at java.awt.LightweightDispatcher.dispatchEvent(Container.java:3095)	at java.awt.Container.dispatchEventImpl(Container.java:1609)	at java.awt.Window.dispatchEventImpl(Window.java:1590)	at java.awt.Component.dispatchEvent(Component.java:3480)	at java.awt.EventQueue.dispatchEvent(EventQueue.java:450)	at java.awt.EventDispatchThread.pumpOneEventForHierarchy(EventDispatchThread.java:197)	at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:150)	at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:144)	at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:136)	at java.awt.EventDispatchThread.run(EventDispatchThread.java:99)