Exception Thrown On Pool.close() While Debugging, But Not While Running

I don't think I encountered this problem working on this in Python 2.7, but while debugging in 3.7, Python throws an exception when pool.close() is called. This is the relevant par

Solution 1:

Which PyCharm version do you use? This seems to be fixed in 2019.1.2 by https://youtrack.jetbrains.com/issue/PY-34436