Null object returned when "Discard Legend"

org.apache.wicket.WicketRuntimeException: Null object returned for expression: legend.onlineResource for setting value: on: StyleInfoImpl[wind:mystyle] at org.apache.wicket.core.util.lang.PropertyResolver.setValue(PropertyResolver.java:148) at org.apache.wicket.model.AbstractPropertyModel.setObject(AbstractPropertyModel.java:131) at org.apache.wicket.Component.setDefaultModelObject(Component.java:3082) at org.apache.wicket.markup.html.form.FormComponent.setModelObject(FormComponent.java:1579) at org.geoserver.wms.web.data.ExternalGraphicPanel$5.onSubmit(ExternalGraphicPanel.java:250) at org.apache.wicket.ajax.markup.html.form.AjaxButton$1.onSubmit(AjaxButton.java:113) at org.apache.wicket.ajax.form.AjaxFormSubmitBehavior$AjaxFormSubmitter.onSubmit(AjaxFormSubmitBehavior.java:215) at org.apache.wicket.markup.html.form.Form.delegateSubmit(Form.java:1307) at org.apache.wicket.markup.html.form.Form.process(Form.java:974) at org.apache.wicket.markup.html.form.Form.onFormSubmitted(Form.java:795) at org.apache.wicket.ajax.form.AjaxFormSubmitBehavior.onEvent(AjaxFormSubmitBehavior.java:171) at org.apache.wicket.ajax.AjaxEventBehavior.respond(AjaxEventBehavior.java:155) at org.apache.wicket.ajax.AbstractDefaultAjaxBehavior.onRequest(AbstractDefaultAjaxBehavior.java:588) at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62) at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) at java.lang.reflect.Method.invoke(Method.java:498) at org.apache.wicket.RequestListenerInterface.internalInvoke(RequestListenerInterface.java:258) at org.apache.wicket.RequestListenerInterface.invoke(RequestListenerInterface.java:241) at org.apache.wicket.core.request.handler.ListenerInterfaceRequestHandler.invokeListener(ListenerInterfaceRequestHandler.java:248) at org.apache.wicket.core.request.handler.ListenerInterfaceRequestHandler.respond(ListenerInterfaceRequestHandler.java:234) at org.apache.wicket.request.cycle.RequestCycle$HandlerExecutor.respond(RequestCycle.java:865) at org.apache.wicket.request.RequestHandlerStack.execute(RequestHandlerStack.java:64) at org.apache.wicket.request.cycle.RequestCycle.execute(RequestCycle.java:265) at org.apache.wicket.request.cycle.RequestCycle.processRequest(RequestCycle.java:222) at org.apache.wicket.request.cycle.RequestCycle.processRequestAndDetach(RequestCycle.java:293) at org.apache.wicket.protocol.http.WicketFilter.processRequestCycle(WicketFilter.java:261) at org.apache.wicket.protocol.http.WicketFilter.processRequest(WicketFilter.java:203) at org.apache.wicket.protocol.http.WicketServlet.doPost(WicketServlet.java:159) at javax.servlet.http.HttpServlet.service(HttpServlet.java:650) at javax.servlet.http.HttpServlet.service(HttpServlet.java:731) at org.springframework.web.servlet.mvc.ServletWrappingController.handleRequestInternal(ServletWrappingController.java:158) at org.springframework.web.servlet.mvc.AbstractController.handleRequest(AbstractController.java:147) at org.springframework.web.servlet.mvc.SimpleControllerHandlerAdapter.handle(SimpleControllerHandlerAdapter.java:50) at org.springframework.web.servlet.DispatcherServlet.doDispatch(DispatcherServlet.java:959) at org.springframework.web.servlet.DispatcherServlet.doService(DispatcherServlet.java:893) at org.springframework.web.servlet.FrameworkServlet.processRequest(FrameworkServlet.java:968) at org.springframework.web.servlet.FrameworkServlet.doPost(FrameworkServlet.java:870) at javax.servlet.http.HttpServlet.service(HttpServlet.java:650) at org.springframework.web.servlet.FrameworkServlet.service(FrameworkServlet.java:844) at javax.servlet.http.HttpServlet.service(HttpServlet.java:731) at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:303) at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:208) at org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52) at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:241) at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:208) at org.geoserver.filters.ThreadLocalsCleanupFilter.doFilter(ThreadLocalsCleanupFilter.java:28) at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:241) at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:208) at org.geoserver.filters.SpringDelegatingFilter$Chain.doFilter(SpringDelegatingFilter.java:75) at org.geoserver.wms.animate.AnimatorFilter.doFilter(AnimatorFilter.java:71) at org.geoserver.filters.SpringDelegatingFilter$Chain.doFilter(SpringDelegatingFilter.java:71) at org.geoserver.filters.SpringDelegatingFilter.doFilter(SpringDelegatingFilter.java:46) at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:241) at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:208) at org.geoserver.platform.AdvancedDispatchFilter.doFilter(AdvancedDispatchFilter.java:50) at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:241) at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:208) at org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:316) at org.geoserver.security.filter.GeoServerCompositeFilter$NestedFilterChain.doFilter(GeoServerCompositeFilter.java:69) at org.springframework.security.web.access.intercept.FilterSecurityInterceptor.invoke(FilterSecurityInterceptor.java:126) at org.springframework.security.web.access.intercept.FilterSecurityInterceptor.doFilter(FilterSecurityInterceptor.java:90) at org.geoserver.security.filter.GeoServerCompositeFilter$NestedFilterChain.doFilter(GeoServerCompositeFilter.java:73) at org.geoserver.security.filter.GeoServerCompositeFilter.doFilter(GeoServerCompositeFilter.java:92) at org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:330) at org.geoserver.security.filter.GeoServerCompositeFilter$NestedFilterChain.doFilter(GeoServerCompositeFilter.java:69) at org.springframework.security.web.access.ExceptionTranslationFilter.doFilter(ExceptionTranslationFilter.java:114) at org.geoserver.security.filter.GeoServerCompositeFilter$NestedFilterChain.doFilter(GeoServerCompositeFilter.java:73) at org.geoserver.security.filter.GeoServerCompositeFilter.doFilter(GeoServerCompositeFilter.java:92) at org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:330) at org.geoserver.security.filter.GeoServerAnonymousAuthenticationFilter.doFilter(GeoServerAnonymousAuthenticationFilter.java:54) at org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:330) at org.geoserver.security.filter.GeoServerCompositeFilter$NestedFilterChain.doFilter(GeoServerCompositeFilter.java:69) at org.springframework.security.web.authentication.AbstractAuthenticationProcessingFilter.doFilter(AbstractAuthenticationProcessingFilter.java:205) at org.geoserver.security.filter.GeoServerCompositeFilter$NestedFilterChain.doFilter(GeoServerCompositeFilter.java:73) at org.geoserver.security.filter.GeoServerCompositeFilter.doFilter(GeoServerCompositeFilter.java:92) at org.geoserver.security.filter.GeoServerUserNamePasswordAuthenticationFilter.doFilter(GeoServerUserNamePasswordAuthenticationFilter.java:116) at org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:330) at org.geoserver.security.filter.GeoServerCompositeFilter$NestedFilterChain.doFilter(GeoServerCompositeFilter.java:69) at org.springframework.security.web.authentication.rememberme.RememberMeAuthenticationFilter.doFilter(RememberMeAuthenticationFilter.java:157) at org.geoserver.security.filter.GeoServerCompositeFilter$NestedFilterChain.doFilter(GeoServerCompositeFilter.java:73) at org.geoserver.security.filter.GeoServerCompositeFilter.doFilter(GeoServerCompositeFilter.java:92) at org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:330) at org.geoserver.security.filter.GeoServerCompositeFilter$NestedFilterChain.doFilter(GeoServerCompositeFilter.java:69) at org.springframework.security.web.context.SecurityContextPersistenceFilter.doFilter(SecurityContextPersistenceFilter.java:91) at org.geoserver.security.filter.GeoServerSecurityContextPersistenceFilter$1.doFilter(GeoServerSecurityContextPersistenceFilter.java:53) at org.geoserver.security.filter.GeoServerCompositeFilter$NestedFilterChain.doFilter(GeoServerCompositeFilter.java:73) at org.geoserver.security.filter.GeoServerCompositeFilter.doFilter(GeoServerCompositeFilter.java:92) at org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:330) at org.springframework.security.web.FilterChainProxy.doFilterInternal(FilterChainProxy.java:213) at org.springframework.security.web.FilterChainProxy.doFilter(FilterChainProxy.java:176) at org.geoserver.security.GeoServerSecurityFilterChainProxy.doFilter(GeoServerSecurityFilterChainProxy.java:152) at org.springframework.web.filter.DelegatingFilterProxy.invokeDelegate(DelegatingFilterProxy.java:346) at org.springframework.web.filter.DelegatingFilterProxy.doFilter(DelegatingFilterProxy.java:262) at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:241) at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:208) at org.geoserver.filters.LoggingFilter.doFilter(LoggingFilter.java:87) at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:241) at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:208) at org.geoserver.filters.GZIPFilter.doFilter(GZIPFilter.java:42) at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:241) at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:208) at org.geoserver.filters.SessionDebugFilter.doFilter(SessionDebugFilter.java:48) at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:241) at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:208) at org.geoserver.filters.FlushSafeFilter.doFilter(FlushSafeFilter.java:44) at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:241) at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:208) at org.springframework.web.filter.CharacterEncodingFilter.doFilterInternal(CharacterEncodingFilter.java:121) at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:107) at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:241) at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:208) at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:220) at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:122) at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:505) at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:169) at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:103) at org.apache.catalina.valves.AccessLogValve.invoke(AccessLogValve.java:956) at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:116) at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:436) at org.apache.coyote.http11.AbstractHttp11Processor.process(AbstractHttp11Processor.java:1078) at org.apache.coyote.AbstractProtocol$AbstractConnectionHandler.process(AbstractProtocol.java:625) at org.apache.tomcat.util.net.AprEndpoint$SocketProcessor.doRun(AprEndpoint.java:2517) at org.apache.tomcat.util.net.AprEndpoint$SocketProcessor.run(AprEndpoint.java:2506) at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142) at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617) at org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61) at java.lang.Thread.run(Thread.java:745)