Tree of trees

I have a tree of trees. like this
[-]Tree Root (Tree.java)
[-]SubTree1(Tree.java)
Item1-1(TreeItem.java)
[-]SubTree2(Tree.java)
Item2-1(TreeItem.java)
Item2-2(TreeItem.java)
Item2-3(TreeItem.java)
When putting focus on Tree Root and pressing arrow down i can reach
all items By using navigation keys up on tree when trying to select
Item2-1 from Item2-2 i got the following error

java.lang.NullPointerException: null
at com.google.gwt.user.client.ui.Tree.getTopClosedParent(Tree.java:
1048)
at com.google.gwt.user.client.ui.Tree.moveSelectionDown(Tree.java:
1210)
at com.google.gwt.user.client.ui.Tree.keyboardNavigation(Tree.java:
1107)
at com.google.gwt.user.client.ui.Tree.onBrowserEvent(Tree.java:
654)
at com.google.gwt.user.client.DOM.dispatchEventImpl(DOM.java:1321)
at com.google.gwt.user.client.DOM.dispatchEvent(DOM.java:1277)
at sun.reflect.GeneratedMethodAccessor29.invoke(Unknown Source)
at
sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:
25)
at java.lang.reflect.Method.invoke(Method.java:597)
at
com.google.gwt.dev.shell.MethodAdaptor.invoke(MethodAdaptor.java:103)
at
com.google.gwt.dev.shell.MethodDispatch.invoke(MethodDispatch.java:71)
at
com.google.gwt.dev.shell.OophmSessionHandler.invoke(OophmSessionHandler.java:
167)
at
com.google.gwt.dev.shell.BrowserChannelServer.reactToMessagesWhileWaitingForReturn(BrowserChannelServer.java:
326)
at
com.google.gwt.dev.shell.BrowserChannelServer.invokeJavascript(BrowserChannelServer.java:
207)
at
com.google.gwt.dev.shell.ModuleSpaceOOPHM.doInvoke(ModuleSpaceOOPHM.java:
132)
at
com.google.gwt.dev.shell.ModuleSpace.invokeNative(ModuleSpace.java:
561)
at
com.google.gwt.dev.shell.ModuleSpace.invokeNativeObject(ModuleSpace.java:
269)
at
com.google.gwt.dev.shell.JavaScriptHost.invokeNativeObject(JavaScriptHost.java:
91)
at com.google.gwt.core.client.impl.Impl.apply(Impl.java)
at com.google.gwt.core.client.impl.Impl.entry0(Impl.java:214)
at sun.reflect.GeneratedMethodAccessor22.invoke(Unknown Source)
at
sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:
25)
at java.lang.reflect.Method.invoke(Method.java:597)
at
com.google.gwt.dev.shell.MethodAdaptor.invoke(MethodAdaptor.java:103)
at
com.google.gwt.dev.shell.MethodDispatch.invoke(MethodDispatch.java:71)
at
com.google.gwt.dev.shell.OophmSessionHandler.invoke(OophmSessionHandler.java:
167)
at
com.google.gwt.dev.shell.BrowserChannelServer.reactToMessages(BrowserChannelServer.java:
281)
at
com.google.gwt.dev.shell.BrowserChannelServer.processConnection(BrowserChannelServer.java:
531)
at
com.google.gwt.dev.shell.BrowserChannelServer.run(BrowserChannelServer.java:
352)
at java.lang.Thread.run(Thread.java:662)

I expect to explore tree and subtree by using arrows keys

I am using gwt2.3.0 on windows using firefox

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