A NPE exeception is thrown when the following code is used to issue the signal event:
 processService.signalProcessInstance(pi.getId(), ROUTING_EXCEPTION, null);

The stack trace is:

Caused by: java.lang.NullPointerException
	at org.kie.spring.persistence.AbstractKieSpringJpaManager$EmHolderDelegateInvocationHandler.invoke(AbstractKieSpringJpaManager.java:141) [kie-spring-6.3.0.Final-redhat-5.jar:6.3.0.Final-redhat-5]
	at com.sun.proxy.$Proxy67.joinTransaction(Unknown Source)
	at org.jbpm.process.audit.JPAWorkingMemoryDbLogger.joinTransaction(JPAWorkingMemoryDbLogger.java:246) [jbpm-audit-6.3.0.Final-redhat-5.jar:6.3.0.Final-redhat-5]
	at org.jbpm.process.audit.JPAWorkingMemoryDbLogger.persist(JPAWorkingMemoryDbLogger.java:195) [jbpm-audit-6.3.0.Final-redhat-5.jar:6.3.0.Final-redhat-5]
	at org.jbpm.process.audit.JPAWorkingMemoryDbLogger.afterNodeLeft(JPAWorkingMemoryDbLogger.java:120) [jbpm-audit-6.3.0.Final-redhat-5.jar:6.3.0.Final-redhat-5]
	at org.drools.core.event.ProcessEventSupport.fireAfterNodeLeft(ProcessEventSupport.java:124) [drools-core-6.3.0.Final-redhat-5.jar:6.3.0.Final-redhat-5]
	at org.jbpm.workflow.instance.impl.NodeInstanceImpl.triggerNodeInstance(NodeInstanceImpl.java:375) [jbpm-flow-6.3.0.Final-redhat-5.jar:6.3.0.Final-redhat-5]
	at org.jbpm.workflow.instance.impl.NodeInstanceImpl.triggerCompleted(NodeInstanceImpl.java:325) [jbpm-flow-6.3.0.Final-redhat-5.jar:6.3.0.Final-redhat-5]
	at org.jbpm.workflow.instance.node.ActionNodeInstance.triggerCompleted(ActionNodeInstance.java:61) [jbpm-flow-6.3.0.Final-redhat-5.jar:6.3.0.Final-redhat-5]
	at org.jbpm.workflow.instance.node.ActionNodeInstance.internalTrigger(ActionNodeInstance.java:57) [jbpm-flow-6.3.0.Final-redhat-5.jar:6.3.0.Final-redhat-5]
	at org.jbpm.workflow.instance.impl.NodeInstanceImpl.trigger(NodeInstanceImpl.java:173) [jbpm-flow-6.3.0.Final-redhat-5.jar:6.3.0.Final-redhat-5]
	... 50 more


The problem is caused by the current implementation in org.jbpm.kie.services.impl.ProcessServiceImpl.
Note that it disposes the runtime engine after sending the signal event:
      try {
           ksession.signalEvent(signalName, event, processInstanceId);
       } finally {
       	disposeRuntimeEngine(manager, engine);
       }

So the disposeRuntimeEngine needs to be fixed actually on runtime manager level to handle it properly for all possible cases.