I get this error

When someone attacks a player or an animal this happens they get kicked from the server.

Expected: Value was not Null)
  at Oxide.Core.Plugins.CSPlugin.OnCallHook (System.String name, System.Object[] args) [0x00107] in <beb2b64691c64e2b95b99491bd85442c>:0 
  at Oxide.Core.Plugins.Plugin.CallHook (System.String hook, System.Object[] args) [0x00060] in <beb2b64691c64e2b95b99491bd85442c>:0 
AssertionException: connection != null
Assertion failure. Value was Null
Expected: Value was not Null

4 hours ago, jayinwww said:

When someone attacks a player or an animal this happens they get kicked from the server.

Expected: Value was not Null)
  at Oxide.Core.Plugins.CSPlugin.OnCallHook (System.String name, System.Object[] args) [0x00107] in <beb2b64691c64e2b95b99491bd85442c>:0 
  at Oxide.Core.Plugins.Plugin.CallHook (System.String hook, System.Object[] args) [0x00060] in <beb2b64691c64e2b95b99491bd85442c>:0 
AssertionException: connection != null
Assertion failure. Value was Null
Expected: Value was not Null

Neither of these things would have anything to do with NpcSpawn, you named players and animals, NpcSpawn is for spawning NPCs.

I think some of the error is missing but this does not appear to be an issue at all with NpcSpawn.