I'm not exactly sure what triggered this error, I think it might have been the mob I had targeted got a DoT put on it by me using Sunfire on the one next to it.
Does that sound plausible?
I've had it happen a couple of times but when it happens I'm in the middle of an M+ run and I don't have time to work out what the problem was. This time it happened at the start of a boss fight I was healing so I couldn't reload my UI until the end, I was half expecting WoW to crash.

244x EventHorizon\EventHorizon-1.9.24-beta.lua:1566: attempt to perform arithmetic on a nil value
EventHorizon\EventHorizon-1.9.24-beta.lua:1566: in function `UpdateDoT'
EventHorizon\EventHorizon-1.9.24-beta.lua:1306: in function `UNIT_AURA'
EventHorizon\EventHorizon-1.9.24-beta.lua:799: in function `f'
EventHorizon\EventHorizon-1.9.24-beta.lua:699: in function <EventHorizon\EventHorizon.lua:690>

Locals:
InCombatSkipped