NPE in plugin manager

After something like 20 times clicking in the cms node tree:

01.07.2008 10:16:34 ERROR [org.hippoecm.frontend.plugin.impl.PluginManager.unregisterTracker():222] unregistering a listener that wasn't r
egistered.
01.07.2008 10:16:34 ERROR [org.apache.wicket.RequestCycle.logRuntimeException():1432]
java.lang.NullPointerException
at org.hippoecm.frontend.plugin.impl.PluginManager.unregisterService(PluginManager.java:169)
at org.hippoecm.frontend.plugin.impl.PluginContext.unregisterService(PluginContext.java:126)
at org.hippoecm.frontend.model.ModelService.destroy(ModelService.java:52)
at org.hippoecm.frontend.editor.viewer.ViewerPlugin.onModelChanged(ViewerPlugin.java:68)
at org.apache.wicket.Component.modelChanged(Component.java:2106)
at org.apache.wicket.Component.setModel(Component.java:2821)
at org.apache.wicket.MarkupContainer.setModel(MarkupContainer.java:715)
at org.hippoecm.frontend.service.render.RenderService.updateModel(RenderService.java:132)
at org.hippoecm.frontend.model.ModelService.setModel(ModelService.java:63)
at org.hippoecm.frontend.plugins.versioning.VersionPane.onModelChanged(VersionPane.java:153)
at org.apache.wicket.Component.modelChanged(Component.java:2106)
at org.apache.wicket.Component.setModel(Component.java:2821)
at org.apache.wicket.MarkupContainer.setModel(MarkupContainer.java:715)
at org.hippoecm.frontend.service.render.RenderService.updateModel(RenderService.java:132)
at org.hippoecm.frontend.model.ModelService.setModel(ModelService.java:63)
at org.hippoecm.frontend.service.render.RenderService.setModel(RenderService.java:126)
at org.hippoecm.frontend.plugins.cms.browse.list.DocumentListingPlugin.onSelect(DocumentListingPlugin.java:44)
at org.hippoecm.frontend.plugins.cms.browse.list.DocumentListingNodeCell.onSelect(DocumentListingNodeCell.java:37)
at org.hippoecm.frontend.plugins.standards.list.NodeCell$1.onClick(NodeCell.java:53)
at org.apache.wicket.ajax.markup.html.AjaxLink$1.onEvent(AjaxLink.java:65)
at org.apache.wicket.ajax.AjaxEventBehavior.respond(AjaxEventBehavior.java:163)
at org.apache.wicket.ajax.AbstractDefaultAjaxBehavior.onRequest(AbstractDefaultAjaxBehavior.java:293)
at org.apache.wicket.request.target.component.listener.BehaviorRequestTarget.processEvents(BehaviorRequestTarget.java:100)
at org.apache.wicket.request.AbstractRequestCycleProcessor.processEvents(AbstractRequestCycleProcessor.java:91)
at org.apache.wicket.RequestCycle.processEventsAndRespond(RequestCycle.java:1171)
at org.apache.wicket.RequestCycle.step(RequestCycle.java:1248)
at org.apache.wicket.RequestCycle.steps(RequestCycle.java:1349)
at org.apache.wicket.RequestCycle.request(RequestCycle.java:493)
at org.apache.wicket.protocol.http.WicketFilter.doGet(WicketFilter.java:387)
at org.apache.wicket.protocol.http.WicketFilter.doFilter(WicketFilter.java:199)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:235)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:233)
at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:175)
at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:128)
at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:102)
at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:109)
at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:286)
at org.apache.coyote.http11.Http11Processor.process(Http11Processor.java:844)
at org.apache.coyote.http11.Http11Protocol$Http11ConnectionHandler.process(Http11Protocol.java:583)
at org.apache.tomcat.util.net.JIoEndpoint$Worker.run(JIoEndpoint.java:447)
at java.lang.Thread.run(Thread.java:595)