Description of problem:
When you try to add a ProcessEventListener to StatefulKnowledgeSession and you are missing jBPM libraries on classpath, you get a NullPointerException.

Version-Release number of selected component (if applicable):
BRMS-5.3.0.dev5
Drools-5.3.0.Final

How reproducible:
Using attached reproducer.

Steps to Reproduce:
1. Make sure you don't have any jBPM libs on classpath
2. Create stateful session
3. Add ProcesEventListener (e.g. DebugProcessEventListener)
  
Actual results:
java.lang.NullPointerException
	at org.drools.impl.StatefulKnowledgeSessionImpl.addEventListener(StatefulKnowledgeSessionImpl.java:199)
	at org.sample.TryProcessListener.main(TryProcessListener.java:20)

Expected results:
Either meaningful exception (like NojBPMException) or correct addition of listener

Additional info:
It only affects StatefulKnowledgeSession, the StatefulKnowledgeSession runs just fine.