null pointer exception when building very simple processes with multiple instance subprocess

I have created a reproducer ([https://github.com/mauriziocarioli/multiple-instance-subprocess-reproducer|https://github.com/mauriziocarioli/multiple-instance-subprocess-reproducer)] with four bpmn files:

• a.bpmn and b.bpmn have a multiple instance subprocess and build (mvn clean install) fails with a npe.
• a2.bpmn and b2.bpmn have the subprocess removed and both build successfully

Since multiple instance subprocesses are supported I suspect either a bug or misconfiguration. The bpmn's are very simple so it should not be hard to find what causes the npe.