I'm doing some model manipulation in the UI like removing and adding item on a listbox and getting this error once in a while which does not seems to be coming from my code.

SEVERE: >>java.lang.NullPointerException
>> at java.util.AbstractCollection.addAll(AbstractCollection.java:333)
>> at org.zkoss.bind.tracker.impl.TrackerImpl.getAllTrackerNodesByBean0(TrackerImpl.java:408)
>> at org.zkoss.bind.tracker.impl.TrackerImpl.getAllTrackerNodesByBean(TrackerImpl.java:400)
>> at org.zkoss.bind.tracker.impl.TrackerImpl.collectLoadBindings(TrackerImpl.java:164)
>> at org.zkoss.bind.tracker.impl.TrackerImpl.collectLoadBindings(TrackerImpl.java:181)
>> at org.zkoss.bind.tracker.impl.TrackerImpl.getLoadBindings(TrackerImpl.java:146)
>> at org.zkoss.bind.impl.BinderImpl.loadOnPropertyChange(BinderImpl.java:368)
>> at org.zkoss.bind.impl.BinderImpl.access$100(BinderImpl.java:94)
>> at org.zkoss.bind.impl.BinderImpl$QueueListener.onEvent(BinderImpl.java:236)
>> at org.zkoss.zk.ui.event.impl.DesktopEventQueue$QueueListener.onEvent(DesktopEventQueue.java:145)
>> at org.zkoss.zk.ui.AbstractComponent.onEvent(AbstractComponent.java:2736)
>> at org.zkoss.zk.ui.AbstractComponent.service(AbstractComponent.java:2707)
>> at org.zkoss.zk.ui.AbstractComponent.service(AbstractComponent.java:2648)
>> at org.zkoss.zk.ui.impl.EventProcessor.process(EventProcessor.java:136)
>> at org.zkoss.zk.ui.impl.UiEngineImpl.processEvent(UiEngineImpl.java:1702)
>> at org.zkoss.zk.ui.impl.UiEngineImpl.process(UiEngineImpl.java:1487)
>> at org.zkoss.zk.ui.impl.UiEngineImpl.execUpdate(UiEngineImpl.java:1204)
>> at org.zkoss.zk.au.http.DHtmlUpdateServlet.process(DHtmlUpdateServlet.java:558)
>> at org.zkoss.zk.au.http.DHtmlUpdateServlet.doGet(DHtmlUpdateServlet.java:456)
>> at org.zkoss.zk.au.http.DHtmlUpdateServlet.doPost(DHtmlUpdateServlet.java:464)
>> at javax.servlet.http.HttpServlet.service(HttpServlet.java:641)
>> at javax.servlet.http.HttpServlet.service(HttpServlet.java:722)
>> at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:305)
>> at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:210)
>> at org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:368)
>> at org.springframework.security.web.access.intercept.FilterSecurityInterceptor.invoke(FilterSecurityInterceptor.java:109)
>> at org.springframework.security.web.access.intercept.FilterSecurityInterceptor.doFilter(FilterSecurityInterceptor.java:83)
>> at org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:380)
>> at org.springframework.security.web.access.ExceptionTranslationFilter.doFilter(ExceptionTranslationFilter.java:97)
>> at org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:380)
>> at org.springframework.security.web.session.SessionManagementFilter.doFilter(SessionManagementFilter.java:100)
>> at org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:380)
>> at org.springframework.security.web.authentication.AnonymousAuthenticationFilter.doFilter(AnonymousAuthenticationFilter.java:78)
>> at org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:380)
>> at org.springframework.security.web.servletapi.SecurityContextHolderAwareRequestFilter.doFilter(SecurityContextHolderAwareRequestFilter.java:54)
>> at org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:380)
>> at org.springframework.security.web.savedrequest.RequestCacheAwareFilter.doFilter(RequestCacheAwareFilter.java:35)
>> at org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:380)
>> at org.springframework.security.web.authentication.AbstractAuthenticationProcessingFilter.doFilter(AbstractAuthenticationProcessingFilter.java:187)
>> at org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:380)
>> at org.springframework.security.web.authentication.logout.LogoutFilter.doFilter(LogoutFilter.java:105)
>> at org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:380)
>> at org.springframework.security.web.context.SecurityContextPersistenceFilter.doFilter(SecurityContextPersistenceFilter.java:79)
>> at org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:380)
>> at org.springframework.security.web.session.ConcurrentSessionFilter.doFilter(ConcurrentSessionFilter.java:109)
>> at org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:380)
>> at org.springframework.security.web.FilterChainProxy.doFilter(FilterChainProxy.java:169)
>> at org.springframework.web.filter.DelegatingFilterProxy.invokeDelegate(DelegatingFilterProxy.java:346)
>> at org.springframework.web.filter.DelegatingFilterProxy.doFilter(DelegatingFilterProxy.java:259)
>> at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:243)
>> at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:210)
>> at org.springframework.orm.jpa.support.OpenEntityManagerInViewFilter.doFilterInternal(OpenEntityManagerInViewFilter.java:180)
>> at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:107)
>> at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:243)
>> at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:210)
>> at org.springframework.web.filter.HiddenHttpMethodFilter.doFilterInternal(HiddenHttpMethodFilter.java:77)
>> at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:107)
>> at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:243)
>> at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:210)
>> at org.springframework.web.filter.CharacterEncodingFilter.doFilterInternal(CharacterEncodingFilter.java:88)
>> at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:107)
>> at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:243)
>> at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:210)
>> at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:224)
>> at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:169)
>> at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:472)
>> at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:168)
>> at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:98)
>> at org.apache.catalina.valves.AccessLogValve.invoke(AccessLogValve.java:927)
>> at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:118)
>> at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:407)
>> at org.apache.coyote.http11.AbstractHttp11Processor.process(AbstractHttp11Processor.java:987)
>> at org.apache.coyote.AbstractProtocol$AbstractConnectionHandler.process(AbstractProtocol.java:579)
>> at org.apache.tomcat.util.net.JIoEndpoint$SocketProcessor.run(JIoEndpoint.java:309)
>> at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1110)
>> at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:603)
>> at java.lang.Thread.run(Thread.java:722)