NullReferenceException

Failed to call hook 'OnBossKilled' on plugin 'SkillTreeItemsPro v1.0.5' (NullReferenceException: Object reference not set to an instance of an object.)
  at Oxide.Plugins.SkillTreeItemsPro.RollForDropContainer (BasePlayer player, System.UInt64 skinID, System.String identifier, Oxide.Plugins.SkillTreeItemsPro+LootSource source, UnityEngine.Vector3 position, System.Single heightMod) [0x00039] in <b4bb5c518dc4444285b9e11586b8389d>:0 
  at Oxide.Plugins.SkillTreeItemsPro.OnBossKilled (ScientistNPC npc, BasePlayer attacker) [0x0001a] in <b4bb5c518dc4444285b9e11586b8389d>:0 
  at Oxide.Plugins.SkillTreeItemsPro.DirectCallHook (System.String name, System.Object& ret, System.Object[] args) [0x00fee] in <b4bb5c518dc4444285b9e11586b8389d>: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 

Got this when I killed a boss from the Boss Monster plugin

Yeah that's odd. The attacker or boss is null.

I will add some null checks on the hook in the next release.