this happens when reloading an unrelated plugin XDQuest. reloading GUIShop fixes the problem but it is still a problem.

Failed to call hook 'OnEnterNPC' on plugin 'GUIShop v2.3.15' (NullReferenceException: Object reference not set to an instance of an object)
  at Oxide.Core.Libraries.Permission.UserHasPermission (System.String id, System.String perm) [0x0000a] in <12678b905a6d43c3a9cc366104306651>:0
  at Oxide.Plugins.GUIShop.OnEnterNPC (BasePlayer npc, BasePlayer player) [0x0003d] in <7025e01b319a4216959cfbae1048e9b5>:0
  at Oxide.Plugins.GUIShop.DirectCallHook (System.String name, System.Object& ret, System.Object[] args) [0x01918] in <7025e01b319a4216959cfbae1048e9b5>:0
                                                                                                                                                                                                                                                                                                               2kb/s in, 35kb/s out
  at Oxide.Plugins.CSharpPlugin.InvokeMethod (Oxide.Core.Plugins.HookMethod method, System.Object[] args) [0x00079] in <60c318df79ed41688ea59335e48d61ad>:0
  at Oxide.Core.Plugins.CSPlugin.OnCallHook (System.String name, System.Object[] args) [0x000d8] in <12678b905a6d43c3a9cc366104306651>:0
  at Oxide.Core.Plugins.Plugin.CallHook (System.String hook, System.Object[] args) [0x00060] in <12678b905a6d43c3a9cc366104306651>:0

* This release Update 2.3.16 
* Updated to ignore 3rd party plugin NPCs trying to access the shops.