NullPointerException or IllegalStateException occurs when session.execute() are called concurrently with BRMS 6

Issue

If session.execute is called concurrently with BRMS 6, NullpinterException or IllegalStateExeition occurs as follows:

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)

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)