Also, try changing the parenting from
Lua Code:
local Incoming = CreateFrame("Frame", nil, PARENT_FRAME, BackdropTemplateMixin and "BackdropTemplate");
to
Lua Code:
local Incoming = CreateFrame("Frame", nil, LocationMessageView, BackdropTemplateMixin and "BackdropTemplate");
Unless you have PARENT_FRAME assigned as something somewhere, it is nil.
The name of your XML frame (LocationMessageView) will be added to the global table and, assuming your XML file/frame loads first, you can use that as the parent in your .lua frame. This is done with the names of all frames.