Crash when parenting QWebView to QMainWindow

When attempting to parent a QWebView widget to a QMainWindow that already has a QWindowContainer widget in its hierarchy, the application crashes when subsequently calling APIs like raise()/close() on the main window.

1. Create a QMainWindow instance.
2. Create a QWindowContainer widget and add it to the QMainWindow's layout.
3. Create a QWebView widget.
4. Parent the QWebView widget to the QMainWindow.
5. Attempt to call APIs like {{raise() }}on the QMainWindow.
6. Observe the crash.

Note:

• Crash can also be observed when calling any other APIs that involve accessing through the window container's QWindow handle (which was destroyed during the parenting step earlier)
• If this widget is dockable, then docking onto the QMainWindow via drop, drop action would also result in the crash.
• Crash was tested on MacOS but its likely to happen in other platforms as well.
• reproducer_app.zip added to reproduce this.