QWidgets.pyi indicates that the return type of window() is optional:

def window(self) -> PySide6.QtWidgets.QWidget | None: ...

By contrast, I think by construction of QWidget. window() can never return None: either QWidget has no parent, in which case it becomes a window and window() returns itself; or QWidget has a parent, which is another QWidget for which one of the above is true.

from PySide6.QtWidgets import QWidget, QApplication
app = QApplication()
widget = QWidget(None)
print(widget.window())