NPE in JcrSessionModel

java.lang.NullPointerException
at org.hippoecm.frontend.model.JcrSessionModel.detach(JcrSessionModel.java:115)
at org.hippoecm.frontend.session.UserSession.releaseJcrSession(UserSession.java:198)
at org.hippoecm.frontend.plugins.login.LoginPlugin$SignInForm.onDetach(LoginPlugin.java:184)
at org.apache.wicket.Component.detach(Component.java:1160)
at org.apache.wicket.MarkupContainer.detachChildren(MarkupContainer.java:1654)
at org.apache.wicket.Component.detach(Component.java:1180)
at org.apache.wicket.MarkupContainer.detachChildren(MarkupContainer.java:1654)
at org.apache.wicket.Component.detach(Component.java:1180)
at org.apache.wicket.request.target.component.BookmarkablePageRequestTarget.detach(BookmarkablePageRequestTarget.java:142)
at org.apache.wicket.RequestCycle.detach(RequestCycle.java:1122)
at org.apache.wicket.RequestCycle.steps(RequestCycle.java:1454)
at org.apache.wicket.RequestCycle.request(RequestCycle.java:545)
at org.apache.wicket.protocol.http.WicketFilter.doGet(WicketFilter.java:479)
at org.apache.wicket.protocol.http.WicketFilter.doFilter(WicketFilter.java:312)
at org.mortbay.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1157)
at org.hippoecm.frontend.plugins.login.ConcurrentLoginFilter.doFilter(ConcurrentLoginFilter.java:54)
at org.mortbay.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1157)
at org.mortbay.jetty.servlet.ServletHandler.handle(ServletHandler.java:388)
at org.mortbay.jetty.security.SecurityHandler.handle(SecurityHandler.java:216)
at org.mortbay.jetty.servlet.SessionHandler.handle(SessionHandler.java:182)
at org.mortbay.jetty.handler.ContextHandler.handle(ContextHandler.java:765)
at org.mortbay.jetty.webapp.WebAppContext.handle(WebAppContext.java:418)
at org.mortbay.jetty.handler.ContextHandlerCollection.handle(ContextHandlerCollection.java:230)
at org.mortbay.jetty.handler.HandlerCollection.handle(HandlerCollection.java:114)
at org.mortbay.jetty.handler.HandlerWrapper.handle(HandlerWrapper.java:152)
at org.mortbay.jetty.Server.handle(Server.java:326)
at org.mortbay.jetty.HttpConnection.handleRequest(HttpConnection.java:542)
at org.mortbay.jetty.HttpConnection$RequestHandler.headerComplete(HttpConnection.java:923)
at org.mortbay.jetty.HttpParser.parseNext(HttpParser.java:547)
at org.mortbay.jetty.HttpParser.parseAvailable(HttpParser.java:212)
at org.mortbay.jetty.HttpConnection.handle(HttpConnection.java:404)
at org.mortbay.io.nio.SelectChannelEndPoint.run(SelectChannelEndPoint.java:409)
at org.mortbay.thread.QueuedThreadPool$PoolThread.run(QueuedThreadPool.java:582)