is this ZoS or an addon LUA error ?

[Anceane]

2017-05-23T14:08:44.492-05:00 |cff0000Lua Error: EsoUI/Ingame/PlayerProgressBar/PlayerProgressBar.lua:592: attempt to index a nil value
stack traceback:
	EsoUI/Ingame/PlayerProgressBar/PlayerProgressBar.lua:592: in function 'PlayerProgressBar:UpdateBar'
	EsoUI/Ingame/PlayerProgressBar/PlayerProgressBar.lua:886: in function 'PlayerProgressBar:OnBarLevelChange'
	EsoUI/Ingame/PlayerProgressBar/PlayerProgressBar.lua:365: in function 'onLevelChangedCallback'
	EsoUI/Libraries/ZO_Templates/StatusBarTemplates.lua:174: in function 'ZO_WrappingStatusBar:SetValue'
	EsoUI/Ingame/PlayerProgressBar/PlayerProgressBar.lua:523: in function 'PlayerProgressBar:SetBarValue'
	EsoUI/Ingame/PlayerProgressBar/PlayerProgressBar.lua:791: in function 'PlayerProgressBar:AnimateFillIncrease'
	EsoUI/Ingame/PlayerProgressBar/PlayerProgressBar.lua:774: in function 'PlayerProgressBar:OnWaitBeforeFillComplete'
	EsoUI/Ingame/PlayerProgressBar/PlayerProgressBar.lua:395: in function 'func'
	EsoUI/Libraries/Globals/globalapi.lua:195: in function '(anonymous)'|r

Popped in Cyrodill but never in other area.

Any idea please ?
I was on cyrodill. Only time it popped. Never in other areas.