The AccumulateNode.getFirstMatch() throws NPE when some already collected event expires. The problem occurs when thread A is running StatefulKnowledgeSession.fireUntilHalt() while thread B is inserting new events into the session.

The problem occurs when @expires is defined on the event. The lower the @expires value, the more likely the NPE occurs.

I added the testEventExpirationDuringAccumulate() to the CepEspTest that reproduces this issue.