On Retail, 10.1.7, using CW v3.4.2...this happens when 1st logging in on a character . I recall another addon having this kind of error in 10.1.5, and I believe it had to do with the addon doing this type of call before professions registered or something like that. I can't remember the name of the other addon that was having this problem.

2x ...rfaceCaerdonWardrobe/features/TradeSkill.lua:17: attempt to index global 'ProfessionsFrame' (a nil value)
[string "@CaerdonWardrobe/features/TradeSkill.lua"]:17: in function handler' [string "@CaerdonWardrobe/CaerdonWardrobe.lua"]:861: in function <CaerdonWardrobe/CaerdonWardrobe.lua:852> [string "=[C]"]: in function OpenTradeSkill'
[string "@Molinari/hack.lua"]:61: in function <Molinari/hack.lua:54>

Locals:
self =

{
Init = defined @CaerdonWardrobe/features/TradeSkill.lua:10
OnSchematicFormInit = defined @CaerdonWardrobe/features/TradeSkill.lua:60
Refresh = defined @CaerdonWardrobe/features/TradeSkill.lua:84
GetName = defined @CaerdonWardrobe/features/TradeSkill.lua:6
GetTooltipData = defined @CaerdonWardrobe/features/TradeSkill.lua:80
isHooked = true
OnInitializedFrame = defined @CaerdonWardrobe/features/TradeSkill.lua:23
TRADE_SKILL_SHOW = defined @CaerdonWardrobe/features/TradeSkill.lua:14
GetDisplayInfoInternal = defined @CaerdonWardrobe/FeatureMixin.lua:32
IsSameItem = defined @CaerdonWardrobe/FeatureMixin.lua:24
OnUpdate = defined @CaerdonWardrobe/FeatureMixin.lua:78
GetDisplayInfo = defined @CaerdonWardrobe/features/TradeSkill.lua:87
}
name = nil
(*temporary) = defined @SharedXML/Scroll/ScrollUtil.lua:38
(*temporary) = nil
(*temporary) = nil
(*temporary) = nil
(*temporary) = nil
(*temporary) = "attempt to index global 'ProfessionsFrame' (a nil value)"

Looks like the same issue CanIMogIt was having here: p3lim-wow/Molinari#111

I've identified the fix and will land it shortly!