Created attachment 984087 [details]
reproducer

Description of problem:
If session.execute is called concurrently, NullpinterException or IllegalStateExeition occurs. 


Steps to Reproduce:
1. Download and import Drools6ReoccurNPEProject.zip to JBDS
2. Fix jars and JVM option(like: -Xms1303m -Xmx1303m)
3. Run com.sample.DroolsTest

Actual results:
One of following types of Exception occurs.
===
java.lang.IllegalMonitorStateException
	at java.util.concurrent.locks.ReentrantLock$Sync.tryRelease(ReentrantLock.java:155)
	at java.util.concurrent.locks.AbstractQueuedSynchronizer.release(AbstractQueuedSynchronizer.java:1260)
	at java.util.concurrent.locks.ReentrantLock.unlock(ReentrantLock.java:460)
	at org.drools.core.common.AbstractWorkingMemory.endBatchExecution(AbstractWorkingMemory.java:1753)
	at org.drools.core.impl.StatelessKnowledgeSessionImpl.execute(StatelessKnowledgeSessionImpl.java:381)
	at com.sample.RuleTestThread.execRun(RuleTestThread.java:39)
	at com.sample.RuleTestThread.run(RuleTestThread.java:23)
===

===
java.lang.NullPointerException
	at org.drools.core.impl.StatelessKnowledgeSessionImpl.execute(StatelessKnowledgeSessionImpl.java:381)
	at com.sample.RuleTestThread.execRun(RuleTestThread.java:39)
	at com.sample.RuleTestThread.run(RuleTestThread.java:23)
===

Expected results:
The program can run without any Exception


Additional info:
Our customer shares fixed StatelessKnowledgeSessionImpl.java which is the one in BRMS 6.0.3. Please check their modification. The fixed class is attached as StatelessKnowledgeSessionImpl.CHAGEDraft.java