Calls to QWidget.parent() invalidate non-owning Python objects

When a QWidget that has been inserted in a QTabWidget gets its parent() (or parentWidget()) method called, non-owning Python objects [that are wrapping Qt objects that are present in the widget's hierarchy] are wrongly invalidated.

The issue manifests when the widget is inserted in a QTabWidget, but not with other organizer widgets such as QSplitter. The intermediate QStackedWidget in QTabWidget may be contributing to the issue.

In the following snippet, assume that a "menu" object exists in a menu bar inside the widget (self) which lives in a tab in a QTabWidget:

menu = self.findChild(QtCore.QObject, 'menu')

print(Shiboken.isValid(menu))  # Prints "True"
self.parent()                  # Wrongly invalidates `menu`
print(Shiboken.isValid(menu))  # Prints "False"

An example Qt application (test_app.py) has been attached to easily reproduce the unexpected behavior.