Fix possible segfault when exiting a screen when using the Qt painter

This changes MythScreenStack::PopScreen() to call deleteLater() on the
screen rather than immediately deleting it so as to allow any keypressEvent
handlers to complete safely before the screen is deleted when control
returns to the event loop.

Fixes #11383.

mythtv/libs/libmythui/mythscreenstack.cpp

@@ -128,7 +128,7 @@ void MythScreenStack::PopScreen(MythScreenType *screen, bool allowFade,
                 m_Children.remove(i);
         }
         if (deleteScreen)
-            delete screen;
+            screen->deleteLater();
 
         screen = NULL;