JDK-8322250 demonstrates a case where ThreadReference.ThreadGroup returned 0 for the ThreadGroup, which means there is no ThreadGroup associated with the thread. This is because the thread has terminated. The debug agent uses JVMTI ThreadInfo to get a Thread's ThreadGroup, and w.r.t. the ThreadGroup returned, the ThreadInfo spec says:

 "The thread group to which this thread belongs. NULL if the thread has terminated."

So this is how JDWP ThreadReference.ThreadGroup ends up returning 0, which is something the test doesn't expect. The JDWP and JDI specs should clarify this. No implementation change is needed.