Fix: Subtitle in RadioModalSettingsTile was not changing after more than two opens.

[Juggler98]

This commit solved this and this, but there was still a little problem with not updating subtitle in RadioModalSettingsTile after more than two opens. Check the video.

But I do not know, if this line was important for some other stuff like handling memory leaks, so merge with caution.

[alberto-paiva]

I changed it to:

@override
  void dispose() {
    if (mounted == false) {
      _notifiers.remove(cacheKey);
    }
    super.dispose();
  }

and it works ok here.

[Juggler98]

I changed it to:

@override
  void dispose() {
    if (mounted == false) {
      _notifiers.remove(cacheKey);
    }
    super.dispose();
  }

and it works ok here.

I updated my pull request with this code. It works fine. @GAM3RG33K could you please merge this little change, so the plugin is working properly for everyone? :)