When I try to commit, I run into a NullPointerException that occurs in the PersistentSet when the method isSnapshotEmpty is called. It seems that the snapshot that is checked in this method is set to null during the flush.

Here is the Stacktrace:
at my.company.infrastructure.dataprovider.DataProviderFactory.closeSessionGracefully(DataProviderFactory.java:120)
at my.company.webservice.core.BaseWebservice.closeAllSessionsGracefully(BaseWebservice.java:185)
at my.company.webservice.soap.standard.service.repair.RepairOrderService.searchCurrentRepairOrders(RepairOrderService.java:879)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:606)
at org.jboss.as.ee.component.ManagedReferenceMethodInterceptorFactory$ManagedReferenceMethodInterceptor.processInvocation(ManagedReferenceMethodInterceptorFactory.java:72)
at org.jboss.invocation.InterceptorContext.proceed(InterceptorContext.java:288)
at org.jboss.invocation.WeavedInterceptor.processInvocation(WeavedInterceptor.java:53)
at org.jboss.as.ee.component.interceptors.UserInterceptorFactory$1.processInvocation(UserInterceptorFactory.java:58)
at org.jboss.invocation.InterceptorContext.proceed(InterceptorContext.java:288)
at org.jboss.invocation.WeavedInterceptor.processInvocation(WeavedInterceptor.java:53)
at org.jboss.as.ee.component.interceptors.UserInterceptorFactory$1.processInvocation(UserInterceptorFactory.java:58)
at org.jboss.invocation.InterceptorContext.proceed(InterceptorContext.java:288)
at org.jboss.invocation.InitialInterceptor.processInvocation(InitialInterceptor.java:21)
at org.jboss.invocation.InterceptorContext.proceed(InterceptorContext.java:288)
at org.jboss.invocation.ChainedInterceptor.processInvocation(ChainedInterceptor.java:61)
at org.jboss.as.ee.component.interceptors.ComponentDispatcherInterceptor.processInvocation(ComponentDispatcherInterceptor.java:53)
at org.jboss.invocation.InterceptorContext.proceed(InterceptorContext.java:288)
at org.jboss.as.webservices.deployers.WSComponentInstanceAssociationInterceptor.processInvocation(WSComponentInstanceAssociationInterceptor.java:49)
at org.jboss.invocation.InterceptorContext.proceed(InterceptorContext.java:288)
at org.jboss.invocation.ChainedInterceptor.processInvocation(ChainedInterceptor.java:61)
at org.jboss.as.ee.component.ViewService$View.invoke(ViewService.java:165)
at org.jboss.as.webservices.invocation.AbstractInvocationHandler.invoke(AbstractInvocationHandler.java:112)
at org.jboss.wsf.stack.cxf.JBossWSInvoker.performInvocation(JBossWSInvoker.java:149)
at org.apache.cxf.service.invoker.AbstractInvoker.invoke(AbstractInvoker.java:96)
at org.apache.cxf.jaxws.AbstractJAXWSMethodInvoker.invoke(AbstractJAXWSMethodInvoker.java:178)
at org.apache.cxf.jaxws.JAXWSMethodInvoker.invoke(JAXWSMethodInvoker.java:68)
at org.jboss.wsf.stack.cxf.JBossWSInvoker.invoke(JBossWSInvoker.java:129)
at org.apache.cxf.interceptor.ServiceInvokerInterceptor$1.run(ServiceInvokerInterceptor.java:57)
at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:471)
at java.util.concurrent.FutureTask.run(FutureTask.java:262)
at org.apache.cxf.workqueue.SynchronousExecutor.execute(SynchronousExecutor.java:37)
at org.apache.cxf.interceptor.ServiceInvokerInterceptor.handleMessage(ServiceInvokerInterceptor.java:106)
at org.apache.cxf.phase.PhaseInterceptorChain.doIntercept(PhaseInterceptorChain.java:272)
at org.apache.cxf.transport.ChainInitiationObserver.onMessage(ChainInitiationObserver.java:121)
at org.apache.cxf.transport.http.AbstractHTTPDestination.invoke(AbstractHTTPDestination.java:239)
at org.jboss.wsf.stack.cxf.RequestHandlerImpl.handleHttpRequest(RequestHandlerImpl.java:92)
at org.jboss.wsf.stack.cxf.transport.ServletHelper.callRequestHandler(ServletHelper.java:143)
at org.jboss.wsf.stack.cxf.CXFServletExt.invoke(CXFServletExt.java:87)
at org.apache.cxf.transport.servlet.AbstractHTTPServlet.handleRequest(AbstractHTTPServlet.java:286)
at org.apache.cxf.transport.servlet.AbstractHTTPServlet.doPost(AbstractHTTPServlet.java:206)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:754)
at org.jboss.wsf.stack.cxf.CXFServletExt.service(CXFServletExt.java:135)
at org.jboss.wsf.spi.deployment.WSFServlet.service(WSFServlet.java:140)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:847)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:295)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:214)
at my.company.webservice.logging.LogFilter.handleRequestWithLogging(LogFilter.java:156)
at my.company.webservice.logging.LogFilter.doFilter(LogFilter.java:98)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:246)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:214)
at my.company.webservice.core.RequestCleanupFilter.doFilter(RequestCleanupFilter.java:45)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:246)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:214)
at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:230)
at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:149)
at org.jboss.as.web.security.SecurityContextAssociationValve.invoke(SecurityContextAssociationValve.java:169)
at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:145)
at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:97)
at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:102)
at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:336)
at org.apache.coyote.http11.Http11Processor.process(Http11Processor.java:856)
at org.apache.coyote.http11.Http11Protocol$Http11ConnectionHandler.process(Http11Protocol.java:653)
at org.apache.tomcat.util.net.JIoEndpoint$Worker.run(JIoEndpoint.java:920)
at java.lang.Thread.run(Thread.java:744)
Caused by: java.lang.NullPointerException
at org.hibernate.collection.internal.PersistentSet.isSnapshotEmpty(PersistentSet.java:129)
at org.hibernate.engine.spi.CollectionEntry.isSnapshotEmpty(CollectionEntry.java:405)
at org.hibernate.event.internal.AbstractFlushingEventListener.flushCollections(AbstractFlushingEventListener.java:285)
at org.hibernate.event.internal.AbstractFlushingEventListener.flushEverythingToExecutions(AbstractFlushingEventListener.java:103)
at org.hibernate.event.internal.DefaultFlushEventListener.onFlush(DefaultFlushEventListener.java:55)
at org.hibernate.internal.SessionImpl.flush(SessionImpl.java:1222)
at org.hibernate.internal.SessionImpl.managedFlush(SessionImpl.java:425)
at org.hibernate.engine.transaction.internal.jdbc.JdbcTransaction.beforeTransactionCommit(JdbcTransaction.java:101)
at org.hibernate.engine.transaction.spi.AbstractTransactionImpl.commit(AbstractTransactionImpl.java:177)
at my.company.webservice.hibernate.dataprovider.JEEHibernateDataProviderSession.commit(JEEHibernateDataProviderSession.java:32)
at my.company.infrastructure.dataprovider.DataProviderFactory.closeSessionGracefully(DataProviderFactory.java:118)
... 66 more