Not sure what the issue is, but I get the following error whenever I have auto-sell enabled and open a vendor:

https://imgur.com/a/0ayYydR

So I managed to reproduce the error once with APR but I don't get the error message after a /reload
But I had the same error with BagBrother, so I think it's a change on Blizzard API

I confirm WoW has changed the function in their API, we need to readapt the code on our side
https://wowpedia.fandom.com/wiki/Patch_10.0.2/API_changes