npc buy with backpacks

[andrekinas]

if 'buy with backpacks' is enabled generates this error, and not finish the purchase

Lua Script Error: [Npc interface](Unknown scriptfile)
data/npc/lib/npc.lua:25: attempt to call method 'addItem' (a nil value)
stack traceback:
[C]: in function 'addItem'
data/npc/lib/npc.lua:25: in function 'doNpcSellItem'
data/npc/lib/npcsystem/modules.lua:1048: in function 'callbackOnBuy'
data/npc/lib/npcsystem/npchandler.lua:267: in function 'processModuleCallback'
data/npc/lib/npcsystem/npchandler.lua:471: in function 'onBuy'
data/npc/lib/npcsystem/modules.lua:1144: in function <data/npc/lib/npcsystem/modules.lua:1144>