iOS: Restore mirror mode using [UIScreen mainScreen] instead of nil

The latter had the same effect, but is an invalid assignment for a value
that is documented to be non-nil.

Change-Id: Ice00482138481556ad839bc3721e577dafcb26fe
Reviewed-by: Richard Moe Gustavsen <richard.gustavsen@theqtcompany.com>
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@theqtcompany.com>

src/plugins/platforms/ios/qiosviewcontroller.mm

@@ -143,7 +143,7 @@
 
     if (uiWindow.screen != [UIScreen mainScreen] && self.subviews.count == 1) {
         // Removing the last view of an external screen, go back to mirror mode
-        uiWindow.screen = nil;
+        uiWindow.screen = [UIScreen mainScreen];
         uiWindow.hidden = YES;
     }
 }