I think the general idea of the problem has been stated, but I figured I
would state the official issue.  When Python begins to shutdown it takes
each module and sets each variable in the global namespace to None. If a
thread has not terminated before the interpreter terminates then the
thread tries to use a global variable which has been set to None.

This is not about to change since this occurs because of coding
"errors". You must make sure that either your thread is as safe as a
__del__ method (which means no global namespace access) or you can't let
the app exit until you are positive all of your threads have terminated,
not just asked them to shutdown since this is all asynchronous.