NOT A NEW ISSUE, it seems to occur after every WoW update. I really appreciate the addon and hope you can get it working right.
**Which software were you running W11

• Addon version name: PetTracker 11.0.9
• Client used: retail most current version

Have you read the changelog?
Mark if read. I would if I could easily FIND THE CHANGELOG.

Please describe the bug.
Instant LUA error on enabling addon.
Message: Interface/AddOns/PetTracker/addons/main/main.lua:69: attempt to index field 'sets' (a nil value)
Time: Mon Dec 30 07:03:43 2024
Count: 1
Stack: Interface/AddOns/PetTracker/addons/main/main.lua:69: attempt to index field 'sets' (a nil value)
[string "@Interface/AddOns/Blizzard_ExpansionLandingPage/Blizzard_WarWithinLandingPage.lua"]:48: in function CreateOverlay' [string "@Interface/AddOns/Blizzard_ExpansionLandingPage/Blizzard_ExpansionLandingPage.lua"]:82: in function RefreshExpansionOverlay'
[string "@Interface/AddOns/Blizzard_ExpansionLandingPage/Blizzard_ExpansionLandingPage.lua"]:37: in function <...pansionLandingPage/Blizzard_ExpansionLandingPage.lua:35>

Locals: parent = Frame {
WarWithinLandingOverlay = Frame {
}
}
overlayFrame = nil

Please describe how to reproduce it.
Enable addon. Reload. LUA Error immediate, resolved after disabling addon.

Any Screenshots?
If applicable, please add screenshots to help explain your problem.

Error Logs are Important!
Type /console scriptErrors 1 in the chat and reload the game. If an error window appears when reproducing the bug, please write here the first message (labelled 1/Many) that appeared. ⚠️ By default, the last error message is shown, not the first! You need to navigate back to the first error.

I am having the same issue.

2x PetTracker/addons/main/main.lua:69: attempt to index field 'sets' (a nil value)
[string "@PetTracker/addons/main/main.lua"]:69: in function <PetTracker/addons/main/main.lua:68>
[string "=(tail call)"]: ?
[string "@Blizzard_Menu/DropdownButton.lua"]:32: in function op' [string "@Blizzard_Menu/MenuUtil.lua"]:38: in function <Blizzard_Menu/MenuUtil.lua:36> [string "@Blizzard_Menu/MenuUtil.lua"]:56: in function TraverseMenu'
[string "@Blizzard_Menu/DropdownButton.lua"]:27: in function <Blizzard_Menu/DropdownButton.lua:17>
[string "@Blizzard_Menu/DropdownButton.lua"]:285: in function SignalUpdate' [string "@Blizzard_Menu/Menu.lua"]:2558: in function ModifyMenu'
[string "@AngrierWorldQuests/Modules/QuestFrame/QuestFrameModule.lua"]:1012: in function OverrideShouldShowQuest' [string "@AngrierWorldQuests/Modules/QuestFrame/QuestFrameModule.lua"]:1034: in function <...rWorldQuests/Modules/QuestFrame/QuestFrameModule.lua:1033> [string "=[C]"]: ? [string "@Ace3/AceAddon-3.0-13/AceAddon-3.0.lua"]:66: in function <Ace3/AceAddon-3.0/AceAddon-3.0.lua:61> ... [string "=[C]"]: in function UIParentLoadAddOn'
[string "@Blizzard_UIParent/Mainline/UIParent.lua"]:628: in function MajorFactions_LoadUI' [string "@Blizzard_ExpansionLandingPage/Blizzard_DragonflightLandingPage.lua"]:103: in function SetUpMajorFactionList'
[string "@Blizzard_ExpansionLandingPage/Blizzard_DragonflightLandingPage.lua"]:96: in function RefreshMajorFactionList' [string "@Blizzard_ExpansionLandingPage/Blizzard_DragonflightLandingPage.lua"]:91: in function RefreshOverlay'
[string "@Blizzard_ExpansionLandingPage/Blizzard_DragonflightLandingPage.lua"]:83: in function <...sionLandingPage/Blizzard_DragonflightLandingPage.lua:79>
[string "=[C]"]: in function CreateFrame' [string "@Blizzard_ExpansionLandingPage/Blizzard_DragonflightLandingPage.lua"]:49: in function CreateOverlay'
[string "@MissionReportButtonPlus/MissionReportButtonPlus.lua"]:1588: in function `RefreshExpansionOverlay'
[string "@Blizzard_ExpansionLandingPage/Blizzard_ExpansionLandingPage.lua"]:37: in function <...pansionLandingPage/Blizzard_ExpansionLandingPage.lua:35>

Locals:
key = "showSpecies"
(*temporary) = nil
(*temporary) = "attempt to index field 'sets' (a nil value)"
Addon =

Is anyone monitoring or updating this Very Useful Addon?
It's perfect for completionists or even for those who just want plenty of battle pets, but it's useless when every click causes the huge LUA error to pop up.

The turnaround time for fixes are quiet high.
Do to this (and some threatening issues due to the license used to be "almost open source, but not at all"), I have created another addon, which has most of the same features as this one, but doesn't have these issues.

If you are interrested, it is called Battle Pet Completionist

You're welcome :)