I encountered the following error several times today:

Message: Interface\AddOns\AllTheThings\AllTheThings.lua:9399: attempt to perform arithmetic on field '?' (a nil value)
Time: Fri Aug 21 22:01:54 2020
Count: 22
Stack: Interface\AddOns\AllTheThings\AllTheThings.lua:9399: attempt to perform arithmetic on field '?' (a nil value)
[string "=[C]"]: ?
[string "@Interface\AddOns\AllTheThings\AllTheThings.lua"]:9399: in function <Interface\AddOns\AllTheThings\AllTheThings.lua:9002>

Locals: 

I'm using version AllTheThings-2_0_6_Release.zip, which I believe is the latest at the time of writing this issue.

I'm 99% sure it's a duplicate of #503.
Does it happen when you mouseover something in the list? Happens to me when I mouseover some drops in normal Firelands on my hunter in BM spec.

I'm pretty sure I wasn't mousing over anything in the list when it happened. I was just farming some transmogs from Hellfire Citadel and this error occurred maybe 3 times. I tried mousing over stuff in the list now and I couldn't reproduce it that way. Maybe I have to be mousing over some specific part of the list?

On the other hand, it does look like it's the same issue, so close this one as duplicate if you want. :)

Tell me your class, spec and difficulty of HC you got these errors in, please.

Demon hunter; Havoc; Mythic (pretty sure I got one in Heroic though);

Also, I don't know whether this helps, but I started seeing this error after I turned in https://www.wowhead.com/quest=39364 (it's started by an item that drops from Hellfire Citadel), but this may have been just a coincidence.

This will be fixed in next release, for now you can download current in-development version with fix here.