Fix strong mode error in platform_channel/main.dart (#15624)

* Fix strong mode error in platform_channel/main.dart

* Addres code review comments.

examples/platform_channel/lib/main.dart

@@ -47,7 +47,7 @@ class _PlatformChannelState extends State<PlatformChannel> {
     });
   }
 
-  void _onError(PlatformException error) {
+  void _onError(Object error) {
     setState(() {
       _chargingStatus = 'Battery status: unknown.';
     });