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I'll leave the debug trace for the experts to figure out. No parts of the given debug code have been modified and remain completely untouched from the first installation of Namelessmc v2, in other words, the User.php file has never been modified since I installed Namelessmc v2.
Debug Trace: https://debug.namelessmc.com/QMd3ViSV5T
In short, deleting a user that is currently active (logged in and validated through email) results in a TypeError and crashes the site for a good 4 minutes with the bug before it resolves itself, probably finally processed deleting the user completely and revisiting the broken code and proceeds as normal.
Thanks for the report and the debug link - it looks like there is something like a widget somewhere which is still looking for the deleted user thanks to invalid cache data. As you noticed, it should sort itself out after a few minutes once the cache expires.
We will aim to fix the issue in an upcoming release.