Expected Behavior

To have the addon work without errors

Actual Behavior

The following LUA error pops up;

Message: [string "*VendorerFrames.xml:293_OnShow"]:1: attempt to call method 'SetBackdropColor' (a nil value)
Time: Mon Nov 28 23:09:57 2022
Count: 1
Stack: [string "*VendorerFrames.xml:293_OnShow"]:1: attempt to call method 'SetBackdropColor' (a nil value)
[string "*VendorerFrames.xml:293_OnShow"]:1: in function <[string "*VendorerFrames.xml:293_OnShow"]:1>
[string "=[C]"]: in function Show' [string "@Interface/SharedXML/Tooltip/TooltipDataHandler.lua"]:284: in function <Interface/SharedXML/Tooltip/TooltipDataHandler.lua:240> [string "=[C]"]: in function securecallfunction'
[string "@Interface/SharedXML/Tooltip/TooltipDataHandler.lua"]:237: in function <Interface/SharedXML/Tooltip/TooltipDataHandler.lua:236>
[string "=(tail call)"]: ?
[string "@Interface/AddOns/!Vendorer/knownitems.lua"]:56: in function IsItemKnown' [string "@Interface/AddOns/!Vendorer/core.lua"]:1926: in function UpdateMerchantInfo'
[string "@Interface/AddOns/!Vendorer/core.lua"]:1801: in function <Interface/AddOns/!Vendorer/core.lua:1801>
[string "=[C]"]: in function MerchantFrame_UpdateMerchantInfo' [string "@Interface/FrameXML/MerchantFrame.lua"]:142: in function <Interface/FrameXML/MerchantFrame.lua:135> [string "=[C]"]: in function MerchantFrame_Update'
[string "@Interface/AddOns/!Vendorer/vendorfilter.lua"]:566: in function UpdateMerchantItems' [string "@Interface/AddOns/!Vendorer/vendorfilter.lua"]:612: in function RefreshFilter'
[string "@Interface/AddOns/!Vendorer/vendorfilter.lua"]:572: in function ResetFilter' [string "@Interface/AddOns/!Vendorer/core.lua"]:1728: in function ?'
[string "@Interface/AddOns/Ace3/CallbackHandler-1.0/CallbackHandler-1.0.lua"]:114: in function <...Ons/Ace3/CallbackHandler-1.0/CallbackHandler-1.0.lua:114>
[string "=[C]"]: ?
[string "@Interface/AddOns/Ace3/CallbackHandler-1.0/CallbackHandler-1.0.lua"]:24: in function <...Ons/Ace3/CallbackHandler-1.0/CallbackHandler-1.0.lua:20>
[string "@Interface/AddOns/Ace3/CallbackHandler-1.0/CallbackHandler-1.0.lua"]:59: in function `Fire'
[string "@Interface/AddOns/Ace3/AceEvent-3.0/AceEvent-3.0.lua"]:120: in function <Interface/AddOns/Ace3/AceEvent-3.0/AceEvent-3.0.lua:119>

Locals: self = VendorerTooltip {
0 =
SetQuestLogRewardSpell = defined @Interface/SharedXML/Tooltip/TooltipDataHandler.lua:444
SetQuestPartyProgress = defined @Interface/SharedXML/Tooltip/TooltipDataHandler.lua:444
ProcessLineData = defined @Interface/SharedXML/Tooltip/TooltipDataHandler.lua:303
SetPvpTalent = defined @Interface/SharedXML/Tooltip/TooltipDataHandler.lua:444
SetRuneforgeResultItem = defined @Interface/SharedXML/Tooltip/TooltipDataHandler.lua:444
SetHyperlink = defined @Interface/SharedXML/Tooltip/TooltipDataHandler.lua:444
SetUnit = defined @Interface/SharedXML/Tooltip/TooltipDataHandler.lua:444
SetAzeriteEssence = defined @Interface/SharedXML/Tooltip/TooltipDataHandler.lua:444
SetVoidDepositItem = defined @Interface/SharedXML/Tooltip/TooltipDataHandler.lua:444
SetVoidWithdrawalItem = defined @Interface/SharedXML/Tooltip/TooltipDataHandler.lua:444
SetInfoBackdropStyle = defined @Interface/SharedXML/Tooltip/TooltipDataHandler.lua:338
hasMoney = 1
shownMoneyFrames = 1
SetSocketGem = defined @Interface/SharedXML/Tooltip/TooltipDataHandler.lua:444
numMoneyFrames = 1
info =

Steps to Reproduce the Problem

1. Install addon
2. Open vendor
3. Error

Tested on 2 different PC's

You can try this fork I made where I implemented a bunch of fixes for DragonFlight, until the author comes back to merge or implement themselves.
https://github.com/sebid/Vendorer > https://github.com/sebid/Vendorer/archive/refs/heads/master.zip