1x BossesKilledLFR/main.lua:90: attempt to call global 'IsAddOnLoaded' (a nil value)
[string "@BossesKilledLFR/main.lua"]:90: in function CreateButton' [string "@BossesKilledLFR/main.lua"]:47: in function CreateButtons'
[string "@BossesKilledLFR/main.lua"]:13: in function <BossesKilledLFR/main.lua:12>
[string "=[C]"]: ?
[string "=[C]"]: in function Show' [string "@Blizzard_GroupFinder/Mainline/PVEFrame.lua"]:356: in function GroupFinderFrame_ShowGroupFrame'
[string "@Blizzard_GroupFinder/Mainline/PVEFrame.lua"]:376: in function <...ceBlizzard_GroupFinder/Mainline/PVEFrame.lua:374>

Change IsAddOnLoaded to C_AddOns.IsAddOnLoaded.

Seems to work after that change. The UI works again. :)