added an item to shop getting error codes

hi ive added an item to the shop it all goess well and shows up how ever when i go to purchase the item i get this error string how do i fix it

[ShoppyStock] Player Michael Crackson [76561198056038692] has ran ShoppyStock console command with args: shop listing advanced large planter box t3
Failed to call hook 'ShoppyStockConsoleCommand' on plugin 'ShoppyStock v2.1.5' (NullReferenceException: Object reference not set to an instance of an object)
  at Oxide.Plugins.ShoppyStock.ShowShopListingUI (BasePlayer player, System.String listingKey) [0x001f1] in <6741532be5fd44ce8cfc47ab02db2491>:0 
  at Oxide.Plugins.ShoppyStock.ShoppyStockConsoleCommand (ConsoleSystem+Arg arg) [0x006eb] in <6741532be5fd44ce8cfc47ab02db2491>:0 
  at Oxide.Plugins.ShoppyStock.DirectCallHook (System.String name, System.Object& ret, System.Object[] args) [0x04ee4] in <6741532be5fd44ce8cfc47ab02db2491>:0 
  at Oxide.Plugins.CSharpPlugin.InvokeMethod (Oxide.Core.Plugins.HookMethod method, System.Object[] args) [0x00079] in <42f9bedc659b4f4786eb778d3cd58968>:0 
  at Oxide.Core.Plugins.CSPlugin.OnCallHook (System.String name, System.Object[] args) [0x000de] in <15f61ddda771464d8246ebdce8ff4811>:0 
  at Oxide.Core.Plugins.Plugin.CallHook (System.String hook, System.Object[] args) [0x00060] in <15f61ddda771464d8246ebdce8ff4811>:0