NPE when the underlaying model changed and ajax re-appear.

[smallufo]

Hi , I have a simple panel , with a border and a listView :

<span wicket:id="border">
  <wicket:container wicket:id="commentsView">
    <div>
      //comment layout
    </div>
  </wicket:container>
</span>

And the code :

add(border);
ListView<Comment> commentsView = new ListView<Comment>("commentsView" , new
LoadableDetachableModel<List<Comment>>()
{
  @Override
  protected List<Comment> load() {
    return commentDao.getComments(0 , 10);
  }
})
{
  @Override
  protected void populateItem(ListItem<Comment> item) {
    //comment layout
  }
};
border.add(commentsView);
commentsView.setReuseItems(false);

The Panel has a border , with a clickable button , can be used to
collapse/expand the content of the border with AJAX.
Here is my steps :

1. I first collapse the panel .
2. Then open another browser panel , add a comment. (the underlaying data
model is changed)
3. Back to the original panel , expand the panel ...

And I got a NPE :

2010-11-12 05:19:12,661 ERROR wicket.RequestCycle - Exception in rendering
component: [MarkupContainer [Component id = _body]]
org.apache.wicket.WicketRuntimeException: Exception in rendering component:
[MarkupContainer [Component id = _body]]
        at org.apache.wicket.Component.renderComponent(Component.java:2725)
        at
org.apache.wicket.MarkupContainer.onRender(MarkupContainer.java:1538)
        at org.apache.wicket.Component.render(Component.java:2517)
        at
org.apache.wicket.MarkupContainer.renderNext(MarkupContainer.java:1440)
        at
org.apache.wicket.MarkupContainer.renderComponentTagBody(MarkupContainer.java:1603)
        at
org.apache.wicket.MarkupContainer.onComponentTagBody(MarkupContainer.java:1527)
        at org.apache.wicket.Component.renderComponent(Component.java:2686)
        at
org.apache.wicket.MarkupContainer.onRender(MarkupContainer.java:1538)

...

Caused by: java.lang.NullPointerException
        at
org.apache.wicket.markup.html.border.Border$BorderBodyContainer.onComponentTagBody(Border.java:375)
        at org.apache.wicket.Component.renderComponent(Component.java:2686)
        ... 45 more

Can anybody tell me where goes wrong ? Thanks.
Wicket 1.4.13