blender_window garbage collect bug

[hannesdelbeke]

when trying to get the parent from the blender widget python returns an exception instead
please note first time you run this in blender it works, but second time you get error.

from PySide2.QtWidgets import QApplication
app = QApplication.instance()
a = app._blender_window.parent() 

# first time
# <PySide2.QtGui.QWindow(0x1e6a52d5120, name="QWindowContainerClassWindow") at 0x000001E6A66E7340>

# second
# Error: Python: Traceback (most recent call last):
#  File "\Text", line 3, in <module>
# RuntimeError: Internal C++ object (PySide2.QtGui.QWindow) already deleted.

same if you try access the children, or any other attr

Originally posted by @hannesdelbeke in #42 (comment)