SellItem error

[E] 10:51:51 - SellItems(List<String> itemSell, List<string> itemNoSell, List<Enums.WoWItemQuality> itemQuality): System.NullReferenceException: Object reference not set to an instance of an object.
   at wManager.Wow.Helpers.Vendor..(String , String )
   at System.Linq.Enumerable.Aggregate[TSource,TAccumulate](IEnumerable`1 source, TAccumulate seed, Func`3 func)
   at wManager.Wow.Helpers.Vendor.SellItems(List`1 itemSell, List`1 itemNoSell, List`1 itemQuality, Boolean quickSell)

Bag items:

Tel'Abim Banana  (ID= 4537 )
Razormane Backstabber  (ID= 5093 )
Fractured Canine  (ID= 3299 )
Steel Arrowhead  (ID= 5123 )
Broken Bloodstained Bow  (ID= 4878 )
Spider's Silk  (ID= 3182 )
Crisp Spider Meat  (ID= 1081 )
Gooey Spider Leg  (ID= 2251 )
Bloody Bear Paw  (ID= 2940 )
Big Bear Meat  (ID= 3730 )
Bear Meat  (ID= 3173 )
Bear Gall Bladder  (ID= 3702 )
Lion Meat  (ID= 3731 )
Broken Scorpid Leg  (ID= 4867 )
Melon Juice  (ID= 1205 )
Spellbinder Pants  (ID= 2970 )
Forked Tongue  (ID= 6444 )
Brackwater Vest  (ID= 3306 )
Seeping Gizzard  (ID= 5133 )
Soft Patch of Fur  (ID= 3402 )
Thunder Lizard Tail  (ID= 5470 )
Chunk of Boar Meat  (ID= 769 )
Plans: Copper Chain Vest  (ID= 3609 )
Slimy Murloc Scale  (ID= 5784 )
Small Barnacled Clam  (ID= 5523 )
Murloc Eye  (ID= 730 )
Raw Longjaw Mud Snapper  (ID= 6289 )
Stringy Wolf Meat  (ID= 2672 )
Discolored Worg Heart  (ID= 3164 )
Hearthstone  (ID= 6948 )
Earth Totem  (ID= 5175 )
Fire Totem  (ID= 5176 )
Large Flat Tooth  (ID= 5118 )
Filled Red Waterskin  (ID= 7771 )
Shed Lizard Skin  (ID= 5128 )
Long Tail Hair  (ID= 5120 )