NRE

Failed to call hook 'OnEntityTakeDamage' on plugin 'BossMonster v2.2.1' (NullReferenceException: Object reference not set to an instance of an object.)
  at Oxide.Plugins.BossMonster.OnEntityTakeDamage (ScientistNPC entity, HitInfo info) [0x00020] in <6d0ae9c698024ee89c03236815ca5321>:0 
  at Oxide.Plugins.BossMonster.DirectCallHook (System.String name, System.Object& ret, System.Object[] args) [0x009bb] in <6d0ae9c698024ee89c03236815ca5321>: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 
NullReferenceException: Object reference not set to an instance of an object

The likely cause was accessing entity.net.ID when entity.net was null. 

Unsure the cause, found this in server log and thought id report it

Hello, thanks, and you are right.
Its already in roadmap, you can add this check:
 

        private object OnEntityTakeDamage(ScientistNPC entity, HitInfo info)
        {
            //if (entity == null || info == null) return null;
            if (entity == null || info == null || entity.net == null) return null;
            if (_controllers.ContainsKey(entity.net.ID.Value))
            {

Many thanks for a prompt reply.