prevent window parent from being garbage collected

[hannesdelbeke]

this fixes the bug where if you run this twice, it errors the second time

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.