typing: QTreeWidget.topLevelItem() may return None

According to documenation : https://doc.qt.io/qt-6/qtreewidget.html#topLevelItem

QTreeWidget.topLevelItem() may return None if the item is not found.

But current types describes the return value as QTreeWidgetItem instead of Optional[QTreeWidgetItem]

The current  typing can lead to incorrect code, where the developers assumes (with the support of type-checking) that an actual widget is always returned, and forgets to check for None. This is precisely the kind of errors for which we want to use typing, to not miss a potential None

(reported on PySide6-stubs)