fix: prevent null check error in _changeSequence when showcase is dismissed during animation

[Copilot]

_ids can be set to null by dismiss() → _cleanupAfterSteps() while _onComplete()'s reverse animations are still running. When the .then() callback fires afterward, _ids!.length throws TypeError: Null check operator used on a null value.

Fix: add an early-return guard for _ids == null inside the .then() callback, after the existing !_mounted check:

_onComplete().then((_) {
  if (!_mounted) return;
  _activeWidgetId = id;

  // Guard: dismiss() may have set _ids to null while _onComplete() was async
  if (_ids == null) return;

  if (_activeWidgetId! >= _ids!.length || _activeWidgetId!.isNegative) {
    ...
  }
});

Checklist

• The title of my PR starts with a Conventional Commit prefix (fix:, feat:, docs: etc).
• I have followed the Contributor Guide when preparing my PR.
• I have updated/added tests for ALL new/updated/fixed functionality.
• I have updated/added relevant documentation in docs and added dartdoc comments with ///.
• I have updated/added relevant examples in examples or docs.

Breaking Change?

• Yes, this PR is a breaking change.
• No, this PR is not a breaking change.

Related Issues

Closes #629

[djpirra]

Please proceed with this to the official release :)