一个关于JTree 的问题

是做类似QQ的服务器端 从数据库检索出用户,然后建立一个treemodel,用对象流 将treemode 发送到客户端,客户端接收后 调用jtree.setmodel(treemodel)  更新好友树
张开后是这样的
-好友
|-开发部
|  |__a
|  |__b
|-技术部
| |_c
|现在的问题是 发送过去后,更新tree的时候  客户端 显示
-好友
|-开发部
|-技术部双击开发部,正确的应该可以张开的,但是会报错.
Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
at forms.FriendTreeCellRenderer.getTreeCellRendererComponent(FriendTreePanel.java:119)
at javax.swing.plaf.basic.BasicTreeUI$NodeDimensionsHandler.getNodeDimensions(Unknown Source)
at javax.swing.tree.AbstractLayoutCache.getNodeDimensions(Unknown Source)
at javax.swing.tree.VariableHeightLayoutCache$TreeStateNode.updatePreferredSize(Unknown Source)
at javax.swing.tree.VariableHeightLayoutCache$TreeStateNode.expand(Unknown Source)
at javax.swing.tree.VariableHeightLayoutCache$TreeStateNode.expand(Unknown Source)
at javax.swing.tree.VariableHeightLayoutCache.ensurePathIsExpanded(Unknown Source)
at javax.swing.tree.VariableHeightLayoutCache.setExpandedState(Unknown Source)
at javax.swing.plaf.basic.BasicTreeUI.updateExpandedDescendants(Unknown Source)
at javax.swing.plaf.basic.BasicTreeUI$Handler.treeExpanded(Unknown Source)
at javax.swing.JTree.fireTreeExpanded(Unknown Source)
at javax.swing.JTree.setExpandedState(Unknown Source)
at javax.swing.JTree.expandPath(Unknown Source)
at javax.swing.plaf.basic.BasicTreeUI.toggleExpandState(Unknown Source)
at javax.swing.plaf.basic.BasicTreeUI.selectPathForEvent(Unknown Source)
at javax.swing.plaf.basic.BasicTreeUI$Handler.handleSelectionImpl(Unknown Source)
at javax.swing.plaf.basic.BasicTreeUI$Handler.handleSelection(Unknown Source)
at javax.swing.plaf.basic.BasicTreeUI$Handler.mousePressed(Unknown Source)
at java.awt.AWTEventMulticaster.mousePressed(Unknown Source)
at java.awt.Component.processMouseEvent(Unknown Source)
at javax.swing.JComponent.processMouseEvent(Unknown Source)
at java.awt.Component.processEvent(Unknown Source)
at java.awt.Container.processEvent(Unknown Source)
at java.awt.Component.dispatchEventImpl(Unknown Source)
at java.awt.Container.dispatchEventImpl(Unknown Source)
at java.awt.Component.dispatchEvent(Unknown Source)
at java.awt.LightweightDispatcher.retargetMouseEvent(Unknown Source)
at java.awt.LightweightDispatcher.processMouseEvent(Unknown Source)
at java.awt.LightweightDispatcher.dispatchEvent(Unknown Source)
at java.awt.Container.dispatchEventImpl(Unknown Source)
at java.awt.Window.dispatchEventImpl(Unknown Source)
at java.awt.Component.dispatchEvent(Unknown Source)
at java.awt.EventQueue.dispatchEvent(Unknown Source)
at java.awt.EventDispatchThread.pumpOneEventForHierarchy(Unknown Source)
at java.awt.EventDispatchThread.pumpEventsForHierarchy(Unknown Source)
at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
at java.awt.EventDispatchThread.run(Unknown Source)不知道是何原因treemodel 在服务器端生成jtree 正常.
serMessage = new TreeMessage(UpdateFriend());
JFrame f = new JFrame();
FriendTreePanel ft =new FriendTreePanel();
f.getContentPane().add(ft);
ft.friendTree.setModel(((TreeMessage)serMessage).getTreeModel());
f.setSize(300, 400);
f.setVisible(true);
System.out.println(((TreeMessage)serMessage).getTreeModel());