Noticed I started getting this error right shortly after hitting 81 and only after I started to get a little rest xp. Not sure if you plan on fixing it, but thought I would report it anyways.
Just noticed I have another error that is exactly the same EXCEPT for the event. The event in that one is: event = "PLAYER_XP_UPDATE"
6x nUI-5.07.08 (Plus)\Bars\nUI_XPBar.lua:238: attempt to compare nil with number
nUI-5.07.08 (Plus)\Bars\nUI_XPBar.lua:369: in function <nUI\Bars\nUI_XPBar.lua:340>