WowAce

So with Shadowlands, Chat Copy broke and the layout changed a little.

Count: 1
Message: ..\AddOns\Chatter\Modules\CopyChat.lua line 208:
attempt to index field 'frame' (a nil value)
Debug:
[string "@Chatter\Modules\CopyChat.lua"]:208: Copy()
[string "@Chatter\Modules\CopyChat.lua"]:187: func()
[string "@..\SharedXML\UIDropDownMenu.lua"]:905: UIDropDownMenuButton_OnClick()
[string "*:OnClick"]:1:
[string "*:OnClick"]:1

Message: ..\AddOns\Chatter\Modules\CopyChat.lua line 44:
attempt to call method 'SetBackdrop' (a nil value)
Debug:
[string "@Chatter\Modules\CopyChat.lua"]:44:
Chatter\Modules\CopyChat.lua:39
[string "=[C]"]: ?
[string "@Ace3\AceAddon-3.0\AceAddon-3.0.lua"]:70:
Ace3\AceAddon-3.0\AceAddon-3.0.lua:65
[string "@Ace3\AceAddon-3.0\AceAddon-3.0.lua"]:498: InitializeAddon()
[string "@Ace3\AceAddon-3.0\AceAddon-3.0.lua"]:613:
Ace3\AceAddon-3.0\AceAddon-3.0.lua:605

I believe I was able to fix it by changing line 42 to:

local frame = CreateFrame("Frame", "ChatterCopyFrame", UIParent, BackdropTemplateMixin and "BackdropTemplate")

Indeed, but we then get additional errors in

Message: ...nterface\AddOns\Chatter\Modules\ChatFrameBorders.lua line 248:
attempt to call method 'SetBackdrop' (a nil value)
Debug:
[string "@Chatter\Modules\ChatFrameBorders.lua"]:248: SetBackdrop()
[string "@Chatter\Modules\ChatFrameBorders.lua"]:237: SetBackdrops()
[string "@Chatter\Modules\ChatFrameBorders.lua"]:200: ?()
[string "@Ace3\CallbackHandler-1.0\CallbackHandler-1.0.lua"]:119:
...Ons\Ace3\CallbackHandler-1.0\CallbackHandler-1.0.lua:119
[string "=[C]"]: ?
[string "@Ace3\CallbackHandler-1.0\CallbackHandler-1.0.lua"]:29:
...Ons\Ace3\CallbackHandler-1.0\CallbackHandler-1.0.lua:25
[string "@Ace3\CallbackHandler-1.0\CallbackHandler-1.0.lua"]:64: Fire()
[string "@DBM-Core\Libs\LibSharedMedia-3.0\LibSharedMedia-3.0.lua"]:247: ?()
[string "@Details\boot.lua"]:697: in main chunk
Locals:
None

Message: ..\AddOns\Chatter\Modules\EditBox.lua line 427:
attempt to call method 'SetBackdrop' (a nil value)
Debug:

The first one looks slightly different to the original error.

In Chatter\Modules\ChatFrameBorders.lua -

Change line 57 to:

local frame = CreateFrame("Frame", nil, cf, "ChatFrameBorderTemplate", BackdropTemplateMixin and "BackdropTemplate")

Change line 204 to:

local frame = CreateFrame("Frame", nil, cf, "ChatFrameBorderTemplate", BackdropTemplateMixin and "BackdropTemplate")

Add in this line before line 248:

Mixin(frame, BackdropTemplateMixin)

Should fix the error for you