LUA error on "Fel Regeneration potion"

[SubbyDK]

MTSL TBC: Version 2.5.08

When I click "Fel Regeneration potion" I get this error.

17x ...ceMissingTradeSkillsList_TBC\logic\tools.lua:451: attempt to compare nil with string
[string "@MissingTradeSkillsList_TBC\logic\tools.lua"]:451: in function <...ceMissingTradeSkillsList_TBC\logic\tools.lua:451>
[string "=[C]"]: in function sort' [string "@MissingTradeSkillsList_TBC\logic\tools.lua"]:451: in function <...ceMissingTradeSkillsList_TBC\logic\tools.lua:449> [string "=(tail call)"]: ? [string "@MissingTradeSkillsList_TBC\logic\skill-Skill.lua"]:158: in function HasAtleastOneObtainableItemInZone'
[string "@MissingTradeSkillsList_TBC\logic\skill-Skill.lua"]:48: in function IsSkillAvailableInZone' [string "@MissingTradeSkillsList_TBC\logic\profession.lua"]:100: in function FilterListOfSkills'
[string "@MissingTradeSkillsList_TBC\ui\common\list_frame.lua"]:109: in function UpdateList' [string "@MissingTradeSkillsList_TBC\ui\common\list_frame.lua"]:382: in function RefreshList'
[string "@MissingTradeSkillsList_TBC\ui\common\list_frame.lua"]:371: in function ChangeFilters' [string "@MissingTradeSkillsList_TBC\ui\common\filter_frame.lua"]:712: in function UpdateFilters'
[string "@MissingTradeSkillsList_TBC\ui\common\filter_frame.lua"]:610: in function ChangeContinent' [string "@MissingTradeSkillsList_TBC\ui\common\filter_frame.lua"]:578: in function <...issingTradeSkillsList_TBC\ui\common\filter_frame.lua:575> [string "@MissingTradeSkillsList_TBC\ui\tools.lua"]:355: in function func'
[string "@FrameXML\UIDropDownMenu.lua"]:960: in function `UIDropDownMenuButton_OnClick'
[string ":OnClick"]:1: in function <[string ":OnClick"]:1>

Locals:
a =

{
zone_id = 3520
expansion = 2
name = {
}
xp_level = {
}
id = 20887
phase = 1
reacts = "Hostile"
location = {
}
}
b = {
zone_id = 3520
expansion = 2
name = {
}
xp_level = {
}
id = 19754
phase = 1
reacts = "Hostile"
location = {
}
}
(*temporary) = nil
(*temporary) = "Deathforge Tinkerer"
(*temporary) = "English"
(*temporary) = "attempt to compare nil with string"
property = "name"

[Zereges]

Same issue as #24, fixed by #27 .