Fix crash when exiting while stacked modal dialogs are shown

Remove QWindow instance from QGuiApplicationPrivate::modalWindowList
in the destructor.

Task-number: QTBUG-54566
Change-Id: I1f07fb46a371f69f04907b20657f3b05571445bd
Reviewed-by: Marc Mutz <marc.mutz@kdab.com>
Reviewed-by: Gabriel de Dietrich <gabriel.dedietrich@qt.io>

src/gui/kernel/qwindow.cpp

@@ -204,6 +204,8 @@ QWindow::~QWindow()
 {
     destroy();
     QGuiApplicationPrivate::window_list.removeAll(this);
+    if (!QGuiApplicationPrivate::is_app_closing)
+        QGuiApplicationPrivate::instance()->modalWindowList.removeOne(this);
 }
 
 void QWindowPrivate::init()