rule foo # the problem is usually better "hidden"
when
Object() from accumulate( Object(),
init( Object res = null; )
action( res = null; )
result( res ) )
then
end

causes the exception as shown below, or similar.
A runtime diagnostic should tell the user (at least) that
an accumulate result is null.

ALSO, PLEASE NOTE THIS COMMENT in AccumulateNode.java:642!
// this is a quick hack for the 5.2 release

Exception in thread "main" java.lang.NullPointerException
at org.drools.common.ObjectTypeConfigurationRegistry.getObjectTypeConf(ObjectTypeConfigurationRegistry.java:58)
at org.drools.reteoo.AccumulateNode.evaluateResultConstraints(AccumulateNode.java:646)
at org.drools.reteoo.AccumulateNode.assertLeftTuple(AccumulateNode.java:183)
at org.drools.reteoo.SingleLeftTupleSinkAdapter.doPropagateAssertLeftTuple(SingleLeftTupleSinkAdapter.java:197)
at org.drools.reteoo.SingleLeftTupleSinkAdapter.createAndPropagateAssertLeftTuple(SingleLeftTupleSinkAdapter.java:146)
at org.drools.reteoo.LeftInputAdapterNode.assertObject(LeftInputAdapterNode.java:154)
at org.drools.reteoo.SingleObjectSinkAdapter.propagateAssertObject(SingleObjectSinkAdapter.java:59)
at org.drools.reteoo.ObjectTypeNode.assertObject(ObjectTypeNode.java:204)
at org.drools.reteoo.EntryPointNode.assertObject(EntryPointNode.java:191)
at org.drools.common.NamedEntryPoint.insert(NamedEntryPoint.java:332)
at org.drools.common.NamedEntryPoint.insert(NamedEntryPoint.java:293)
at org.drools.common.AbstractWorkingMemory.insert(AbstractWorkingMemory.java:905)
at org.drools.common.AbstractWorkingMemory.insert(AbstractWorkingMemory.java:864)
at org.drools.impl.StatefulKnowledgeSessionImpl.insert(StatefulKnowledgeSessionImpl.java:255)
at match.Main.makeFacts(Main.java:120)
at match.Main.execute(Main.java:98)
at match.Main.main(Main.java:126)