When editing a loot effect which has a missing class, there's a text in the tooltip saying to press Alt to get more information. Pressing Alt throws an error.

1x ...acetotalRP3_Extended\inventory\container.lua:156: attempt to concatenate field 'id' (a nil value)

Doing it on an existing item throws another related error.

1x ...acetotalRP3_Extended\inventory\container.lua:167: attempt to index local 'rootClass' (a boolean value)

It seems that on the loot editor, there is no slotInfo.id but we have a slotInfo.classID instead.

Checking if slotInfo.id is nil when making the tooltip and use slotInfo.classID instead if it is seems to be fixing the issue according to my tests. I'm not sure it's a pretty fix though, so I'll wait for an opinion before committing to this.