Re-enable platform_channels CI (#1903)

## Pre-launch Checklist

- [x] I read the [Flutter Style Guide] _recently_, and have followed its
advice.
- [x] I signed the [CLA].
- [x] I read the [Contributors Guide].
- [x] I updated/added relevant documentation (doc comments with `///`).
- [x] All existing and new tests are passing.

If you need help, consider asking for advice on the #hackers-devrel
channel on [Discord].

<!-- Links -->
[Flutter Style Guide]:
https://github.com/flutter/flutter/wiki/Style-guide-for-Flutter-repo
[CLA]: https://cla.developers.google.com/
[Discord]: https://github.com/flutter/flutter/wiki/Chat
[Contributors Guide]:
https://github.com/flutter/samples/blob/main/CONTRIBUTING.md

platform_channels/lib/src/method_channel_demo.dart

@@ -46,6 +46,7 @@ class _MethodChannelDemoState extends State<MethodChannelDemo> {
                     final value = await Counter.increment(counterValue: count);
                     setState(() => count = value);
                   } catch (error) {
+                    if (!mounted) return;
                     showErrorMessage(
                       context,
                       (error as PlatformException).message!,
@@ -64,6 +65,7 @@ class _MethodChannelDemoState extends State<MethodChannelDemo> {
                     final value = await Counter.decrement(counterValue: count);
                     setState(() => count = value);
                   } catch (error) {
+                    if (!mounted) return;
                     showErrorMessage(
                       context,
                       (error as PlatformException).message!,