For months now, I've been getting the same error message when I click on an pet icon on the world map:

Message: ...s\PetTracker\addons\main\classes\abstract\specie.lua:26: attempt to index a nil value
Time: Fri Aug 14 18:32:10 2020
Count: 1
Stack: ...s\PetTracker\addons\main\classes\abstract\specie.lua:26: attempt to index a nil value
[string "@interface\AddOns\PetTracker\addons\main\classes\abstract\specie.lua"]:26: in function GetID' [string "@Interface\AddOns\PetTracker\addons\main\classes\abstract\pet.lua"]:33: in function Display'
[string "@interface\AddOns\PetTracker\addons\main\classes\maps\speciePin.lua"]:67: in function <...ns\PetTracker\addons\main\classes\maps\speciePin.lua:65>

Locals: self =

{
specie = 272
}
(*temporary) = nil
(*temporary) = nil
(*temporary) = "attempt to index a nil value"

I have the same problem, since there is no answer since August 14 for this matter I managed to do a "quick fix" for this, I'm not the developer, just trying to help.

Replace lines 33 to 36 of "\PetTracker\addons\main\classes\abstract\pet.lua" with this:
if self:GetSpecie() then
PetJournal_SelectSpecies(PetJournal, self:GetSpecie())
elseif self:GetID() then
PetJournal_SelectPet(PetJournal, self:GetID())

This is caused when you click on a pet that you are missing, it tries to find the best you OWN.