Fix exception thrown in notifier when value is updated after dispose()

[kaboc]

Fixes #48

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (f8e2281) 99.75% compared to head (9df9f89) 99.75%.
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #49      +/-   ##
==========================================
- Coverage   99.75%   99.75%   -0.01%     
==========================================
  Files          16       16              
  Lines         820      819       -1     
==========================================
- Hits          818      817       -1     
  Misses          2        2              

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