NullReferenceException upon load

[BossMonster] File Example has been loaded successfully!
Failed to call hook 'OnServerInitialized' on plugin 'BossMonster v2.2.1' (NullReferenceException: Object reference not set to an instance of an object)
  at Oxide.Plugins.BossMonster.OnServerInitialized () [0x000c5] in <af8df12523054bd1ae268669fb2366bf>:0 
  at Oxide.Plugins.BossMonster.DirectCallHook (System.String name, System.Object& ret, System.Object[] args) [0x006fa] in <af8df12523054bd1ae268669fb2366bf>: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 
Calling 'OnServerInitialized' on 'BossMonster v2.2.1' took 121ms
Loaded plugin BossMonster v2.2.1 by KpucTaJl

Hello again, im writing here just in case for other peoples

How we known from this ticket:
https://codefling.com/files/support/31125-nullreferenceexception-upon-load/

This error appears only once during a hot plugin update; there is no error when the plugin is reloaded or when the server is restarted. This is likely due to the fact that new relationships with NpcSpawn have appeared in the OnServerInitialized method.