Addon version 6.2.0.25
Game version 6.2.2.20490 (x64)

Reproduction scenario

1. Run the game.
2. Find wild pet that you don't have in your collection.
3. Enter pet battle. Surrender.
4. When the pet reappears, mouse-over it.
5. 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]

Same as #8 ... yours is a better report, but theirs had the line number for the current codebase. Fixed.