macOS: Prevent stale references to NSScreen during screen removal

Task-number: QTBUG-57223
Change-Id: I7cdc075a4afee5ad8c23fd3c43a04f2a258b81f9
Reviewed-by: Liang Qi <liang.qi@qt.io>
Reviewed-by: Timur Pocheptsov <timur.pocheptsov@qt.io>
Reviewed-by: Richard Moe Gustavsen <richard.gustavsen@qt.io>

src/plugins/platforms/cocoa/qcocoaintegration.mm

@@ -463,6 +463,8 @@ void QCocoaIntegration::updateScreens()
     // Now the leftovers in remainingScreens are no longer current, so we can delete them.
     foreach (QCocoaScreen* screen, remainingScreens) {
         mScreens.removeOne(screen);
+        // Prevent stale references to NSScreen during destroy
+        screen->m_nsScreen = nil;
         destroyScreen(screen);
     }
 }