Addon version 6.2.0.25
Game version 6.2.2.20490 (x64)
Reproduction scenario
Run the game.
Find wild pet that you don't have in your collection.
Enter pet battle. Surrender.
When the pet reappears, mouse-over it.
Bang, we get an error in Core.lua line 372, can't concatenate nil value.
Fix that worked for me
Replace this line from
infoString = breed .. PetQualityStrings[quality]
to
infoString = (breed or "") .. PetQualityStrings[quality]
Addon version 6.2.0.25 Game version 6.2.2.20490 (x64)
Reproduction scenario
Fix that worked for me Replace this line from
infoString = breed .. PetQualityStrings[quality]
toinfoString = (breed or "") .. PetQualityStrings[quality]