NullPointerException thrown from ShowResultPage

[twagoo]

Currently deployed beta version of VLO-2.18 at ​http://catalog.clarin.eu/vlobeta regularly shows the following exception in its logs:

2014-01-20 11:15:19,812 ERROR [org.apache.wicket.DefaultExceptionMapper#internalMap:123] - Unexpected error occurred
org.apache.wicket.WicketRuntimeException: Can't instantiate page using constructor 'public eu.clarin.cmdi.vlo.pages.ShowResultPage(org.apache.wicket.request.mapper.parameter.PageParameters)' and argument ''. An exception has been thrown during construction!
	at org.apache.wicket.session.DefaultPageFactory.newPage(DefaultPageFactory.java:194)
	at org.apache.wicket.session.DefaultPageFactory.newPage(DefaultPageFactory.java:76)
	at org.apache.wicket.DefaultMapperContext.newPageInstance(DefaultMapperContext.java:133)
	at org.apache.wicket.core.request.handler.PageProvider.resolvePageInstance(PageProvider.java:268)
	at org.apache.wicket.core.request.handler.PageProvider.getPageInstance(PageProvider.java:166)
	at org.apache.wicket.core.request.handler.ListenerInterfaceRequestHandler.getPage(ListenerInterfaceRequestHandler.java:96)
	at org.apache.wicket.core.request.handler.ListenerInterfaceRequestHandler.respond(ListenerInterfaceRequestHandler.java:157)
	at org.apache.wicket.request.cycle.RequestCycle$HandlerExecutor.respond(RequestCycle.java:861)
	at org.apache.wicket.request.RequestHandlerStack.execute(RequestHandlerStack.java:64)
	at org.apache.wicket.request.cycle.RequestCycle.execute(RequestCycle.java:261)
	at org.apache.wicket.request.cycle.RequestCycle.processRequest(RequestCycle.java:218)
	at org.apache.wicket.request.cycle.RequestCycle.processRequestAndDetach(RequestCycle.java:289)
	at org.apache.wicket.protocol.http.WicketFilter.processRequestCycle(WicketFilter.java:259)
	at org.apache.wicket.protocol.http.WicketFilter.processRequest(WicketFilter.java:201)
	at org.apache.wicket.protocol.http.WicketFilter.doFilter(WicketFilter.java:282)
	at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:235)
	at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
	at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:233)
	at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:191)
	at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:127)
	at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:103)
	at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:109)
	at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:293)
	at org.apache.jk.server.JkCoyoteHandler.invoke(JkCoyoteHandler.java:190)
	at org.apache.jk.common.HandlerRequest.invoke(HandlerRequest.java:311)
	at org.apache.jk.common.ChannelSocket.invoke(ChannelSocket.java:776)
	at org.apache.jk.common.ChannelSocket.processConnection(ChannelSocket.java:705)
	at org.apache.jk.common.ChannelSocket$SocketConnection.runIt(ChannelSocket.java:898)
	at org.apache.tomcat.util.threads.ThreadPool$ControlRunnable.run(ThreadPool.java:690)
	at java.lang.Thread.run(Thread.java:744)
Caused by: java.lang.reflect.InvocationTargetException
	at sun.reflect.GeneratedConstructorAccessor13.newInstance(Unknown Source)
	at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:45)
	at java.lang.reflect.Constructor.newInstance(Constructor.java:526)
	at org.apache.wicket.session.DefaultPageFactory.newPage(DefaultPageFactory.java:171)
	... 29 more
Caused by: java.lang.NullPointerException

Which indicates a NullPointerException occurs while the constructor of the ShowResultPage is being executed.

Investigate and prevent from happening.