UIViewComponentPeer: Remove rootViewController from old window before adding it to new window

This fixes an issue where the view would disappear and the window would
become black when the window scene changed, e.g. because screen sharing
was started.

modules/juce_gui_basics/native/juce_UIViewComponentPeer_ios.mm

@@ -577,6 +577,7 @@ private:
             if (@available (iOS 13, *))
                 window.windowScene = nil;
 
+            window.rootViewController = nil;
             [window release];
             window = nil;
         }