QIosScreen: retain the right window

When initializing m_uiWindow (instead of trying to retain this window
itself, which is harmless, since it's nil, but useless, since it's
not the right window).

Pick-to: 6.4 6.2 5.15
Change-Id: I7855fd7c24ebfc28fbb912966f8010caba914fe7
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>

src/plugins/platforms/ios/qiosscreen.mm

@@ -263,7 +263,7 @@ QIOSScreen::QIOSScreen(UIScreen *screen)
     if (!qt_apple_isApplicationExtension()) {
         for (UIWindow *existingWindow in qt_apple_sharedApplication().windows) {
             if (existingWindow.screen == m_uiScreen) {
-                m_uiWindow = [m_uiWindow retain];
+                m_uiWindow = [existingWindow retain];
                 break;
             }
         }