Landing beta changes in master for the new stable release (#747)

platform_design/lib/main.dart

@@ -116,7 +116,7 @@ class _PlatformAdaptingHomePageState extends State<PlatformAdaptingHomePage> {
             );
           default:
             assert(false, 'Unexpected tab');
-            return null;
+            return SizedBox.shrink();
         }
       },
     );