Application crashes when a widget window is processing a mouse event while a non-widget popup window is active

Create an application with:

• widget-based main window
• modeless QML dialog
• Popup with `popupType: Popup.Window` in it

Open the dialog, open the popup, then hover or click the widget window. Application crashes.
`QWidgetWindow::handleMouseEvent()` accesses `QApplication::activePopupWidget()` if `QApplicationPrivate::inPopupMode()` is true, but it checks for an active popup window, which is not necessarily a widget. That scheme is used in several other places.