NPC error

[Paulix]

[3/3/2016 18:35:2] [Error - NpcScript Interface]
[3/3/2016 18:35:2] (Unknown script file)
[3/3/2016 18:35:2] Description:
[3/3/2016 18:35:2] attempt to call a nil value
[3/3/2016 18:35:2] stack traceback:
[3/3/2016 18:35:2]    [C]: in function 'closeShopWindow'
[3/3/2016 18:35:2]    data/npc/lib/npcsystem/npchandler.lua:208: in function 'releaseFocus'
[3/3/2016 18:35:2]    data/npc/lib/npcsystem/npchandler.lua:559: in function 'onWalkAway'
[3/3/2016 18:35:2]    data/npc/lib/npcsystem/npchandler.lua:491: in function 'onThink'
[3/3/2016 18:35:2]    data/npc/scripts/loot.lua:8: in function <data/npc/scripts/loot.lua:8>

any idea? here is npc code

local keywordHandler = KeywordHandler:new()
local npcHandler = NpcHandler:new(keywordHandler)
NpcSystem.parseParameters(npcHandler)

function onCreatureAppear(cid)         npcHandler:onCreatureAppear(cid)        end
function onCreatureDisappear(cid)        npcHandler:onCreatureDisappear(cid)      end
function onCreatureSay(cid, type, msg)      npcHandler:onCreatureSay(cid, type, msg)    end
function onThink()                npcHandler:onThink()              end

-- Don't forget npcHandler = npcHandler in the parameters. It is required for all StdModule functions!
keywordHandler:addKeyword({'helmets'}, StdModule.say, {npcHandler = npcHandler, onlyFocus = true, text = 'I buy royal (40k), warrior (6k), crusader (9k), crown (5k), devil (4k), chain (35gp) and iron helmets (30gp), also mystic turbans (500gp).'})
keywordHandler:addKeyword({'boots'}, StdModule.say, {npcHandler = npcHandler, onlyFocus = true, text = 'I buy golden boots (100k), steel boots (40k) and boots of haste (40k).'})
keywordHandler:addKeyword({'armors'}, StdModule.say, {npcHandler = npcHandler, onlyFocus = true, text = 'I buy golden (30k), crown (20k), knight (5k), lady (7,5k), plate (400gp), brass (200gp) and chain armors (100gp), also mpa (100k), dsm (60k) and blue robes (15k).'})
keywordHandler:addKeyword({'legs'}, StdModule.say, {npcHandler = npcHandler, onlyFocus = true, text = 'I buy golden (80k), crown (15k), knight (6k), plate (500gp) and brass legs (100gp).'})
keywordHandler:addKeyword({'shields'}, StdModule.say, {npcHandler = npcHandler, onlyFocus = true, text = 'I buy blessed (150k), great (100k), demon (40k), vampire (25k), medusa (8k), amazon (4k), crown (5k), tower (4k), dragon (3k), guardian (2k), beholder (1k), and dwarven shields (100gp), also mms (80k).'})
keywordHandler:addKeyword({'swords'}, StdModule.say, {npcHandler = npcHandler, onlyFocus = true, text = 'I buy giant (10k), bright (6k), fire (3k) serpent (1.5k), spike (800gp) and two handed swords (400gp), also ice rapiers (4k), magic longswords (150k), magic swords (90k), warlord swords (100k) broad swords (70gp), short swords (30gp), sabres (25gp) and swords (25gp).'})
keywordHandler:addKeyword({'axes'}, StdModule.say, {npcHandler = npcHandler, onlyFocus = true, text = 'I buy fire (10k), guardian halberds (7,5k) knight (2k), double (200gp) and battle axes (100gp), also dragon lances (10k), stonecutters axes (90k), halberds (200gp) and hatchets (20gp).'})
keywordHandler:addKeyword({'clubs'}, StdModule.say, {npcHandler = npcHandler, onlyFocus = true, text = 'I buy thunder hammers (90k), war (6k), dragon (2k) and battle hammers (60gp), also skull staffs (10k) and clerical maces (200gp).'})

npcHandler:addModule(FocusModule:new())

and here is npchandler.lua
http://pastebin.ca/3387126

i have no idea what triggered this error, probably some player was going to buy/sell some loot