OK just found out how to reproduce the error.
Make shure you don't have raw Jewelcrafting gems in your bag.
Look into guild roster and klick on "show all" at the Jewelcrafting.
Filter to only show gems where you have the mats, now if it shows nothing because you don't have mats the error will pop up:
Code:
10x Juwe-1.0\juwe-1.0.lua:33: attempt to index field "?" (a nil value)
Juwe-1.0\juwe-1.0.lua:33: in function <Juwe\juwe.lua:25>
<in C code>
Blizzard_TradeSkillUI\Blizzard_TradeSkillUI.lua:140: in function <Blizzard_TradeSkillUI\Blizzard_TradeSkillUI.lua:128>
<in C code>
<string>:"*:OnClick":1: in function <string>:"*:OnClick":1
Locals:
(*temporary) = <func> =[C]:-1
= <func> =[C]:-1
= <func> @Juwe\juwe.lua:25
Also Woud be cool to have a small button on the top left of the tradeskill window where I can disable the addon ingame without reloading - this woud e.g. help to search gems that you need for the Jewelcrafting quests.