Don't blow up in msg_created handler if flow has been deleted

[rowanseymour]

No description provided.

[codecov]

Codecov Report

Merging #632 (e1000ec) into main (97df539) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main     #632   +/-   ##
=======================================
  Coverage   61.95%   61.96%           
=======================================
  Files         132      132           
  Lines       12910    12913    +3     
=======================================
+ Hits         7999     8002    +3     
  Misses       4045     4045           
  Partials      866      866           

Impacted Files	Coverage Δ
core/handlers/msg_created.go	75.00% <100.00%> (+1.08%)	⬆️

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