set() doesn't await its file write, so a write in flight when the store is
disposed (app shutdown, or a closing test) would assert on a disposed
ChangeNotifier. Skip the post-write notify once disposed via a _disposed guard +
_safeNotify. Surfaced by the T-293 theme-persistence test flaking under the
loaded parallel run; also the correct behaviour for graceful shutdown.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
jpmschweitzerandClaude Opus 4.8
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