Check video widget is mounted on call to setState (#18467)

When setState() calls occur asynchronously, it's possible that the
Futures on which they're waiting don't complete until the widget is
offscreen. To avoid this, we check the widget is mounted before calling
setState().

examples/flutter_gallery/lib/demo/video_demo.dart

@@ -151,6 +151,7 @@ class _VideoPlayPauseState extends State<VideoPlayPause> {
 
   _VideoPlayPauseState() {
     listener = () {
+      if (mounted)
         setState(() {});
     };
   }
@@ -374,6 +375,7 @@ class _VideoDemoState extends State<VideoDemo>
       controller.play();
       await connectedCompleter.future;
       await controller.initialize();
+      if (mounted)
         setState(() {});
     }