fix: ChannelController.debugAssertNotDisposed() throwing when calli…

[MSOB7YY]

when disposing PlatformInAppWebViewController, it calls InternalChannelController.disposeChannel() which tries to set the channel handler to null, but the issue is that it access the channel through the getter channel which has a debug assert that it shouldn't be disposed (channel == null)

this might or might not cause the resources to not be freed while debugging

in this pr, we access the _channel variable directly, if it's null then setMethodCallHandler will simply not execute

Connection with issue(s)

Resolve issue #2104

Connected to N/A

Testing and Review Notes

to test

  flutter_inappwebview:
    git:
      url: https://github.com/MSOB7YY/flutter_inappwebview
      path: flutter_inappwebview

Screenshots or Videos

To Do

• double check the original issue to confirm it is fully satisfied
• add testing notes and screenshots in PR description to help guide reviewers
• request the "UX" team perform a design review (if/when applicable)

[sabikrahat]

Thank you. This solution works for me.