problem in recursiveTreeNodeAdaptor

[timeoccupy]

i want to build an tree with richfaces through the "recursiveTreeNodeAdaptor". And i added the DragAndDrop in it to realize the function of dragging any elements and dropping them anywhere in the same tree. problem: when i drag one elements and drop it in another,the elements can't be removed in the pre-directory.sometimes a NullPointerException will occur with 'drogValue'. here is my jsp

<rich:tree id="tree" style="width:150px" switchType="ajax">
 <rich:recursiveTreeNodesAdaptor id="treenodeadaptor"
 roots="#{treeBean.points}"
 var="item"
 nodes="#{item.points}"
 includedNode="#{item.render=='real'}">
 <rich:treeNode id="treenode"
 ajaxSingle="true"
 dragIndicator=":indicator"
 acceptedTypes="#{item.type}"
 dragType="dir"
 dragListener="#{treeBean.dragProcess}"
 dropListener="#{treeBean.dropProcess}"
 reRender="tree"
 dragValue="#{item}"
 dropValue="#{item.id}">
 <a4j:commandLink value="#{item.name}" actionListener="#{treeBean.clickProcess}">
 <f:param value="#{item.name}" name="name"></f:param>
 </a4j:commandLink>

 </rich:treeNode>
 </rich:recursiveTreeNodesAdaptor>
</rich:tree>

i build my 'static' tree recursively in a class. If one elements is draged to a dir,its parentID will change,and reBuild the tree

[timeoccupy]

the information in tomcat server:
Failed to re-render tree node: form:tree:treenodeadaptor_:6:treenodeadaptor_:0 due to model data unavailability! Maybe parent node should be re-rendered instead?