ios: Fix crash when closing app while showing a native screen

Fixes: QTBUG-117021
Pick-to: 6.6 6.5
Change-Id: Icebae0e815b42c6e0bf39e727fd1a16db5752d78
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>

src/plugins/platforms/ios/qiosviewcontroller.mm

@@ -107,8 +107,11 @@
 {
     Q_UNUSED(subview);
 
-    Q_ASSERT(self.window);
     UIWindow *uiWindow = self.window;
+    // uiWindow can be null when closing from the ios "app manager" and the app is
+    // showing a native window like UIDocumentBrowserViewController
+    if (!uiWindow)
+        return;
 
     if (uiWindow.screen != [UIScreen mainScreen] && self.subviews.count == 1) {
         // We're about to remove the last view of an external screen, so go back