Failed to call hook 'OnEntityTakeDamage' on plugin 'NTeleportation v1.6.6' (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 <9882f28dc2204b4dba514a9ad18f5042>:0 
  at Oxide.Plugins.NTeleportation.OnEntityTakeDamage (BasePlayer player, HitInfo hitInfo) [0x000bd] in <aa58a370ceb04bb19da4f2bf4cca3851>:0 
  at Oxide.Plugins.NTeleportation.DirectCallHook (System.String name, System.Object& ret, System.Object[] args) [0x005ea] in <aa58a370ceb04bb19da4f2bf4cca3851>:0 
  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 <9882f28dc2204b4dba514a9ad18f5042>:0 
  at Oxide.Core.Plugins.Plugin.CallHook (System.String hook, System.Object[] args) [0x00060] in <9882f28dc2204b4dba514a9ad18f5042>:0

looks like an npc plugin you're using is not assigning UserIDString a value when it spawns the npc.

if (permission.UserHasPermission(player.UserIDString, PermExempt)) return;​

this is where the error happens. there's no reason an exception should be thrown otherwise