4x ...ddOns/PetJournalEnhanced/modules/HookedFunctions.lua:124: attempt to index global 'PetJournalListScrollFrame' (a nil value)
[string "@Interface/AddOns/PetJournalEnhanced/modules/HookedFunctions.lua"]:124: in function <...ddOns/PetJournalEnhanced/modules/HookedFunctions.lua:119>
[string "=[C]"]: ?
[string "@Interface/AddOns/ArkInventory/Externals/Ace3/AceAddon/AceAddon-3.0-13.lua"]:66: in function <...rkInventory/Externals/Ace3/AceAddon/AceAddon-3.0.lua:61>
[string "@Interface/AddOns/ArkInventory/Externals/Ace3/AceAddon/AceAddon-3.0-13.lua"]:494: in function InitializeAddon' [string "@Interface/AddOns/ArkInventory/Externals/Ace3/AceAddon/AceAddon-3.0-13.lua"]:619: in function <...rkInventory/Externals/Ace3/AceAddon/AceAddon-3.0.lua:611> [string "=[C]"]: in function LoadAddOn'
[string "@Interface/AddOns/ArkInventory/Core/ArkInventory.lua"]:11011: in function LoadAddOn' [string "@Interface/AddOns/ArkInventory/Core/ArkInventory.lua"]:11027: in function BlizzardAPIHook'
[string "@Interface/AddOns/ArkInventory/Core/ArkInventory.lua"]:2881: in function <Interface/AddOns/ArkInventory/Core/ArkInventory.lua:2850>
[string "=[C]"]: ?
[string "@Interface/AddOns/ArkInventory/Externals/Ace3/AceAddon/AceAddon-3.0-13.lua"]:66: in function <...rkInventory/Externals/Ace3/AceAddon/AceAddon-3.0.lua:61>
[string "@Interface/AddOns/ArkInventory/Externals/Ace3/AceAddon/AceAddon-3.0-13.lua"]:523: in function `EnableAddon'
[string "@Interface/AddOns/ArkInventory/Externals/Ace3/AceAddon/AceAddon-3.0-13.lua"]:626: in function <...rkInventory/Externals/Ace3/AceAddon/AceAddon-3.0.lua:611>

Another on

20x Interface/SharedXML/HybridScrollFrame.lua:135: attempt to index local 'self' (a nil value)
[string "@Interface/SharedXML/HybridScrollFrame.lua"]:135: in function HybridScrollFrame_GetOffset' [string "@Interface/AddOns/PetJournalEnhanced/modules/HookedFunctions.lua"]:311: in function PetJournal_UpdatePetList'
[string "@Interface/AddOns/PetJournalEnhanced/modules/HookedFunctions.lua"]:29: in function ?' [string "@Interface/AddOns/ArkInventory/Libs/CallBackHandler/CallbackHandler-1.0-7.lua"]:119: in function <...ventory/Libs/CallBackHandler/CallbackHandler-1.0.lua:119> [string "=[C]"]: ? [string "@Interface/AddOns/ArkInventory/Libs/CallBackHandler/CallbackHandler-1.0-7.lua"]:29: in function <...ventory/Libs/CallBackHandler/CallbackHandler-1.0.lua:25> [string "@Interface/AddOns/ArkInventory/Libs/CallBackHandler/CallbackHandler-1.0-7.lua"]:64: in function Fire'
[string "@Interface/AddOns/PetJournalEnhanced/modules/Sorting.lua"]:394: in function SortPets' [string "@Interface/AddOns/PetJournalEnhanced/modules/Sorting.lua"]:259: in function ?'
[string "@Interface/AddOns/ArkInventory/Libs/CallBackHandler/CallbackHandler-1.0-7.lua"]:119: in function <...ventory/Libs/CallBackHandler/CallbackHandler-1.0.lua:119>
[string "=[C]"]: ?
[string "@Interface/AddOns/ArkInventory/Libs/CallBackHandler/CallbackHandler-1.0-7.lua"]:29: in function <...ventory/Libs/CallBackHandler/CallbackHandler-1.0.lua:25>
[string "@Interface/AddOns/ArkInventory/Libs/CallBackHandler/CallbackHandler-1.0-7.lua"]:64: in function Fire' [string "@Interface/AddOns/FishingBuddy/Libs/LibPetJournal-2.0-33/LibPetJournal-2.0.lua"]:336: in function _LoadPetsFinish'
[string "@Interface/AddOns/FishingBuddy/Libs/LibPetJournal-2.0-33/LibPetJournal-2.0.lua"]:263: in function LoadPets' [string "@Interface/AddOns/FishingBuddy/Libs/LibPetJournal-2.0-33/LibPetJournal-2.0.lua"]:380: in function ?'
[string "@Interface/AddOns/FishingBuddy/Libs/LibPetJournal-2.0-33/LibPetJournal-2.0.lua"]:47: in function <...ngBuddy/Libs/LibPetJournal-2.0/LibPetJournal-2.0.lua:46>

And another

13x ...ddOns/PetJournalEnhanced/modules/HookedFunctions.lua:124: attempt to index global 'PetJournalListScrollFrame' (a nil value)
[string "@Interface/AddOns/PetJournalEnhanced/modules/HookedFunctions.lua"]:124: in function <...ddOns/PetJournalEnhanced/modules/HookedFunctions.lua:119>
[string "=[C]"]: ?
[string "@Interface/AddOns/ArkInventory/Externals/Ace3/AceAddon/AceAddon-3.0-13.lua"]:66: in function <...rkInventory/Externals/Ace3/AceAddon/AceAddon-3.0.lua:61>
[string "@Interface/AddOns/ArkInventory/Externals/Ace3/AceAddon/AceAddon-3.0-13.lua"]:494: in function InitializeAddon' [string "@Interface/AddOns/ArkInventory/Externals/Ace3/AceAddon/AceAddon-3.0-13.lua"]:619: in function <...rkInventory/Externals/Ace3/AceAddon/AceAddon-3.0.lua:611> [string "=[C]"]: in function LoadAddOn'
[string "@Interface/AddOns/ArkInventory/Core/ArkInventory.lua"]:11011: in function LoadAddOn' [string "@Interface/AddOns/ArkInventory/Core/ArkInventory.lua"]:11027: in function BlizzardAPIHook'
[string "@Interface/AddOns/ArkInventory/Core/ArkInventory.lua"]:2881: in function <Interface/AddOns/ArkInventory/Core/ArkInventory.lua:2850>
[string "=[C]"]: ?
[string "@Interface/AddOns/ArkInventory/Externals/Ace3/AceAddon/AceAddon-3.0-13.lua"]:66: in function <...rkInventory/Externals/Ace3/AceAddon/AceAddon-3.0.lua:61>
[string "@Interface/AddOns/ArkInventory/Externals/Ace3/AceAddon/AceAddon-3.0-13.lua"]:523: in function `EnableAddon'
[string "@Interface/AddOns/ArkInventory/Externals/Ace3/AceAddon/AceAddon-3.0-13.lua"]:626: in function <...rkInventory/Externals/Ace3/AceAddon/AceAddon-3.0.lua:611>

HookedFunctions.lua:124: attempt to index global 'PetJournalListScrollFrame' (a nil value)
Replace the line PetJournalListScrollFrame:Hide()
By --PetJournalListScrollFrame:Hide()

Seems like the PetJournalListScrollFrame reference is invalid.
Commenting it out hides the error, but double layers the Addon replacement frame with the stock Blizz frame.

Fix:
Inside the HookedFunctions.lua

Replace :
PetJournalListScrollFrame:Hide()

With :
--PetJournalListScrollFrame:Hide()
PetJournal.ScrollBox:Hide()
PetJournal.ScrollBar:Hide()