WildFlyTestRunner throws NullPointerException during stop intermittently

There seems to be a synchronization issue in the WildFly TestRunner (Server class) in the wildfly-core testuite.

I'm getting a NullPointerException intermittently during server stop:

Exception in thread "Thread-5" java.lang.NullPointerException
	at org.wildfly.core.testrunner.Server.lambda$stop$0(Server.java:205)
	at java.lang.Thread.run(Thread.java:748)

This is a testsuite issue which doesn't affect the test results themselves. Nevertheless it would be better to have clean runs without NPEs.