Fixed bug that occurred if a callback attribute used a property which didn't exist yet

[astrofrog]

I might try and add a regression test for this

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.90%. Comparing base (f2cadaf) to head (3f04fa7).
Report is 11 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #50      +/-   ##
==========================================
+ Coverage   96.78%   96.90%   +0.12%     
==========================================
  Files          18       19       +1     
  Lines        2361     2459      +98     
==========================================
+ Hits         2285     2383      +98     
  Misses         76       76              

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