Track controller stream subscription and cancel on dispose

[ltOgt]

Description

// init:
controllerSubscription = widget.controller?.events.listen(_controllerListener);

// dispose:
controllerSubscription?.cancel();

Related Issues

Fixes:

Unhandled Exception: This widget has been unmounted, so the State no longer has a context (and should be considered defunct).
Consider canceling any active work during "dispose" or using the "mounted" getter to determine if the State is still active.

Pre-launch Checklist

• I read the [Contributor Guide] and followed the process outlined there for submitting PRs.
• I updated pubspec.yaml with an appropriate new version according to the [pub versioning philosophy], or this PR is [exempt from version changes].
• I updated CHANGELOG.md to add a description of the change, [following repository CHANGELOG style].
• [-] I updated/added relevant documentation (doc comments with ///).
• All existing and new tests are passing.

Visual reference

No change

[ltOgt]

Not sure why the formatter is still failing the pipeline

[ricardodalarme]

Thank you so much for your contribution! The CI failed because there is a lint issue, could you run dart format .?