As of today's patch, it looks like Blizzard has updated the WoW Classic client's version tag from 11302 to 11303. As a result, this addon no-longer correctly figures out that it's running on Classic, and not Retail. Instead, it fails to run, and displays something like the following error:
XPBarNone\Core.lua:121: attempt to index global 'C_AzeriteItem' (a nil value)
For those who wish to edit the addon's source code manually, there is a simple, one-line fix for this.
The file you want to edit is World of Warcraft\_classic_\Interface\Addons\XPBarNone\Core.lua
On or around line 30 should be something that looks like:
[11302] = true,
Change it to look like the following:
[11303] = true,
Save the file, reload the game, and you should be good.
- WireRydr