Fixed setState issue in MacosPulldownButton

[Abbas1Hussein]

🔄 Reopened PR

This replaces a previous PR that was closed due to a branch issue.
Reopening with a clean branch and the same fix applied.

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🧩 Fix: avoid calling setState after dispose in MacosPulldownButton

When I used MacosPulldownButton, this error appeared during tests:

setState() called after dispose(): _MacosPulldownButtonState (lifecycle state: defunct, not mounted)

It happened because setState was called inside the navigator.push(...).then(...) callback after the widget was already disposed.

I fixed it by adding a mounted check before calling setState:

navigator.push(_pulldownRoute!).then<void>((_) {
  if (!mounted) return;
  setState(() => _pullDownButtonState = PulldownButtonState.enabled);
  _removeMacosPulldownRoute();
});

Thanks for your patience, I’ll be happy to make any additional changes if needed.

[GroovinChip]

LGTM