Fix 'private method 'load' called' when posting non-whitelisted webhook

Closes #221294 (closed)

Testing

There are likely no automated tests which cover this code, since the previous (broken) behavior has been able to sneak in for a long time already.

I have tested this locally, by hand-patching 🙈 the according .haml file locally in our GitLab 14.3.2 instance. With the patch in place, I get this (expected) error instead of the problem described in the ticket (a 500 error with the described stack tracein the production.log file).

MR acceptance checklist

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