Getting NullPointerException at GlobalTimerService

Issue

Following exception intermittently occurs in the logs:

Caused by: java.lang.NullPointerException: null
        at org.jbpm.process.core.timer.impl.GlobalTimerService.getTimerJobInstances(GlobalTimerService.java:149) ~[jbpm-flow-6.0.3-redhat-6.jar:6.0.3-redhat-6]
        at org.jbpm.process.core.timer.impl.RegisteredTimerServiceDelegate.getTimerJobInstances(RegisteredTimerServiceDelegate.java:107) ~[jbpm-flow-6.0.3-redhat-6.jar:6.0.3-redhat-6]
        at org.drools.core.marshalling.impl.ProtobufOutputMarshaller.serializeSession(ProtobufOutputMarshaller.java:215) ~[drools-core-6.0.3-redhat-7.jar:6.0.3-redhat-7]
        at org.drools.core.marshalling.impl.ProtobufOutputMarshaller.writeSession(ProtobufOutputMarshaller.java:121) ~[drools-core-6.0.3-redhat-7.jar:6.0.3-redhat-7]
        at org.drools.core.marshalling.impl.ProtobufMarshaller.marshall(ProtobufMarshaller.java:166) ~[drools-core-6.0.3-redhat-7.jar:6.0.3-redhat-7]
        at org.drools.core.marshalling.impl.ProtobufMarshaller.marshall(ProtobufMarshaller.java:148) ~[drools-core-6.0.3-redhat-7.jar:6.0.3-redhat-7]
        at org.drools.persistence.SessionMarshallingHelper.getSnapshot(SessionMarshallingHelper.java:64) ~[drools-persistence-jpa-6.0.3-redhat-6.jar:6.0.3-redhat-6]
        at org.drools.persistence.info.SessionInfo.update(SessionInfo.java:85) ~[drools-persistence-jpa-6.0.3-redhat-6.jar:6.0.3-redhat-6]
        at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[na:1.7.0_21]
        at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57) ~[na:1.7.0_21]
        at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) ~[na:1.7.0_21]
        at java.lang.reflect.Method.invoke(Method.java:601) ~[na:1.7.0_21]
        at org.hibernate.ejb.event.BeanCallback.invoke(BeanCallback.java:37) ~[hibernate-entitymanager-3.6.7.Final.jar:3.6.7.Final]
        at org.hibernate.ejb.event.EntityCallbackHandler.callback(EntityCallbackHandler.java:94) ~[hibernate-entitymanager-3.6.7.Final.jar:3.6.7.Final]
        at org.hibernate.ejb.event.EntityCallbackHandler.preUpdate(EntityCallbackHandler.java:79) ~[hibernate-entitymanager-3.6.7.Final.jar:3.6.7.Final]
        at org.hibernate.ejb.event.EJB3FlushEntityEventListener.invokeInterceptor(EJB3FlushEntityEventListener.java:61) ~[hibernate-entitymanager-3.6.7.Final.jar:3.6.7.Final]
        at org.hibernate.event.def.DefaultFlushEntityEventListener.handleInterception(DefaultFlushEntityEventListener.java:349) ~[hibernate-core-3.6.7.Final.jar:3.6.7.Final]
        at org.hibernate.event.def.DefaultFlushEntityEventListener.scheduleUpdate(DefaultFlushEntityEventListener.java:287) ~[hibernate-core-3.6.7.Final.jar:3.6.7.Final]
        at org.hibernate.event.def.DefaultFlushEntityEventListener.onFlushEntity(DefaultFlushEntityEventListener.java:155) ~[hibernate-core-3.6.7.Final.jar:3.6.7.Final]
        at org.hibernate.event.def.AbstractFlushingEventListener.flushEntities(AbstractFlushingEventListener.java:219) ~[hibernate-core-3.6.7.Final.jar:3.6.7.Final]
        at org.hibernate.event.def.AbstractFlushingEventListener.flushEverythingToExecutions(AbstractFlushingEventListener.java:99) ~[hibernate-core-3.6.7.Final.jar:3.6.7.Final]
        at org.hibernate.event.def.DefaultFlushEventListener.onFlush(DefaultFlushEventListener.java:50) ~[hibernate-core-3.6.7.Final.jar:3.6.7.Final]
        at org.hibernate.impl.SessionImpl.flush(SessionImpl.java:1216) ~[hibernate-core-3.6.7.Final.jar:3.6.7.Final]
        at org.hibernate.impl.SessionImpl.managedFlush(SessionImpl.java:383) ~[hibernate-core-3.6.7.Final.jar:3.6.7.Final]
        at org.hibernate.transaction.JDBCTransaction.commit(JDBCTransaction.java:133) ~[hibernate-core-3.6.7.Final.jar:3.6.7.Final]
        at org.hibernate.ejb.TransactionImpl.commit(TransactionImpl.java:76) ~[hibernate-entitymanager-3.6.7.Final.jar:3.6.7.Final]
        ... 55 common frames omitted