NullPointerException in console tree rendering

While copying and moving some nodes in the tree following error below is logged.

To reproduce with the testsuite:

• Log in to the console
• Browse to /hst:hst/hst:configurations/demosite/hst:sitemenus/main
• Click on News1 and move it to News (so News1 becomes a child node of News)
• Click on any other node

==> catalina.out <==
26.01.2012 12:17:08 ERROR [org.apache.wicket.RequestCycle.logRuntimeException():1531] Exception in rendering component: [MarkupContainer [Component id = indent]]
org.apache.wicket.WicketRuntimeException: Exception in rendering component: [MarkupContainer [Component id = indent]]
at org.apache.wicket.Component.renderComponent(Component.java:2767)
at org.apache.wicket.MarkupContainer.onRender(MarkupContainer.java:1559)
at org.apache.wicket.Component.render(Component.java:2528)
at org.apache.wicket.MarkupContainer.renderNext(MarkupContainer.java:1461)
at org.apache.wicket.MarkupContainer.renderComponentTagBody(MarkupContainer.java:1624)
at org.apache.wicket.MarkupContainer.onComponentTagBody(MarkupContainer.java:1548)
at org.apache.wicket.Component.renderComponent(Component.java:2728)
at org.apache.wicket.MarkupContainer.onRender(MarkupContainer.java:1559)
at org.apache.wicket.markup.html.tree.AbstractTree$TreeItem.onRender(AbstractTree.java:228)
at org.apache.wicket.Component.render(Component.java:2528)
at org.apache.wicket.Component.renderComponent(Component.java:2669)
at org.apache.wicket.ajax.AjaxRequestTarget.respondComponent(AjaxRequestTarget.java:876)
at org.apache.wicket.ajax.AjaxRequestTarget.respondComponents(AjaxRequestTarget.java:680)
at org.apache.wicket.ajax.AjaxRequestTarget.respond(AjaxRequestTarget.java:590)
at org.apache.wicket.request.AbstractRequestCycleProcessor.respond(AbstractRequestCycleProcessor.java:105)
at org.apache.wicket.RequestCycle.processEventsAndRespond(RequestCycle.java:1260)
at org.apache.wicket.RequestCycle.step(RequestCycle.java:1331)
at org.apache.wicket.RequestCycle.steps(RequestCycle.java:1438)
at org.apache.wicket.RequestCycle.request(RequestCycle.java:546)
at org.apache.wicket.protocol.http.WicketFilter.doGet(WicketFilter.java:486)
at org.apache.wicket.protocol.http.WicketFilter.doFilter(WicketFilter.java:319)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:235)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
at org.apache.wicket.protocol.http.WicketFilter.doFilter(WicketFilter.java:370)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:235)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
at org.hippoecm.frontend.ResourceRewriteFilter.doFilter(ResourceRewriteFilter.java:129)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:235)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
at org.hippoecm.frontend.plugins.login.ConcurrentLoginFilter.doFilter(ConcurrentLoginFilter.java:54)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:235)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:233)
at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:191)
at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:127)
at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:102)
at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:109)
at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:293)
at org.apache.coyote.http11.Http11Processor.process(Http11Processor.java:859)
at org.apache.coyote.http11.Http11Protocol$Http11ConnectionHandler.process(Http11Protocol.java:602)
at org.apache.tomcat.util.net.JIoEndpoint$SocketProcessor.run(JIoEndpoint.java:396)
at java.util.concurrent.ThreadPoolExecutor$Worker.runTask(Unknown Source)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(Unknown Source)
at java.lang.Thread.run(Unknown Source)
Caused by: java.lang.NullPointerException
at org.apache.wicket.extensions.markup.html.tree.DefaultAbstractTree.isNodeLast(DefaultAbstractTree.java:609)
at org.apache.wicket.extensions.markup.html.tree.DefaultAbstractTree.access$000(DefaultAbstractTree.java:50)
at org.apache.wicket.extensions.markup.html.tree.DefaultAbstractTree$1.onComponentTagBody(DefaultAbstractTree.java:301)
at org.apache.wicket.Component.renderComponent(Component.java:2728)
... 43 more