[release-8.4] Fixes 1026106 - in some cases, NSEvent seems to have been null

[monojenkins]

This introduces a null check, and also introduces additional debug levelchecks for others, just to make sure if we do run into this while debugging, that we are aware.

Fixes https://vsmac.dev/1026106

Backport of #9405.

/cc @sandyarmstrong @avodovnik