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

diff --git a/examples/platform_channel/lib/main.dart b/examples/platform_channel/lib/main.dart
index ffae082..69c1f81 100644
--- a/examples/platform_channel/lib/main.dart
+++ b/examples/platform_channel/lib/main.dart
@@ -47,7 +47,7 @@
     });
   }
 
-  void _onError(PlatformException error) {
+  void _onError(Object error) {
     setState(() {
       _chargingStatus = 'Battery status: unknown.';
     });