Regression: Linkpicker doesn't open when trying to browse for a hippo:mirror field

To reproduce (archetype):

• Log in as admin
• Go to the documents, configuration.
• Edit the newsdocument template
• Add the compound field "Link"
• Save and commit the changes
• Browse to Documents and the News folder, and edit a news document
• Click the "[..]" button in the new link field to browse for a file to link to
  Expected result: The linkpicker opens
  Actual result: Nothing happens in the GUI. Errors are logged:

2013-04-11 09:49:43,141 ERROR [http-8080-6] [org.hippoecm.frontend.plugin.config.impl.JcrConfigServiceFactory.getCluster():68] Failed to resolve path relative to node /hippo:configuration/hippo:frontend/cms
2013-04-11 09:49:43,146 ERROR [http-8080-6] [org.apache.wicket.RequestCycle.logRuntimeException():1558]
java.lang.NullPointerException
at org.hippoecm.frontend.plugin.impl.PluginContext.newCluster(PluginContext.java:79)
at org.hippoecm.frontend.plugins.standards.picker.NodePickerController.create(NodePickerController.java:91)
at org.hippoecm.frontend.editor.plugins.linkpicker.LinkPickerDialog.<init>(LinkPickerDialog.java:80)
at org.hippoecm.frontend.editor.plugins.linkpicker.MirrorTemplatePlugin$1.createDialog(MirrorTemplatePlugin.java:63)
at org.hippoecm.frontend.editor.plugins.linkpicker.MirrorTemplatePlugin$1.createDialog(MirrorTemplatePlugin.java:58)
at org.hippoecm.frontend.dialog.DialogAction.execute(DialogAction.java:34)
at org.hippoecm.frontend.dialog.ClearableDialogLink$1.onClick(ClearableDialogLink.java:49)
at org.apache.wicket.ajax.markup.html.AjaxLink$1.onEvent(AjaxLink.java:68)
at org.apache.wicket.ajax.AjaxEventBehavior.respond(AjaxEventBehavior.java:177)
at org.apache.wicket.ajax.AbstractDefaultAjaxBehavior.onRequest(AbstractDefaultAjaxBehavior.java:312)
at org.apache.wicket.request.target.component.listener.BehaviorRequestTarget.processEvents(BehaviorRequestTarget.java:157)
at org.apache.wicket.request.AbstractRequestCycleProcessor.processEvents(AbstractRequestCycleProcessor.java:92)
at org.hippoecm.frontend.PluginRequestCycleProcessor.processEvents(PluginRequestCycleProcessor.java:39)
at org.apache.wicket.RequestCycle.processEventsAndRespond(RequestCycle.java:1279)
at org.apache.wicket.RequestCycle.step(RequestCycle.java:1358)
at org.apache.wicket.RequestCycle.steps(RequestCycle.java:1465)
at org.apache.wicket.RequestCycle.request(RequestCycle.java:545)
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.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.valves.AccessLogValve.invoke(AccessLogValve.java:615)
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$Worker.run(JIoEndpoint.java:489)
at java.lang.Thread.run(Thread.java:722)