Summary

If you click the subplot parameters button from the toolbar, a new window pops up. If you then close the main figure window, the subplot parameters window will still be there and keep the session alive. I would expect the main figure window to close the subplot parameters pop up as well on close. (same experience on macosx and qt5agg)

Proposed fix

Keep a child reference to the subplot parameters window around that we can then close automatically for the user?