showing error while spawning car

it showing this when I spawn a car
but seems it work normally, I am not sure is it effect other plugins.

19:58 [Error] Failed to call hook 'OnEntityKill' on plugin 'SkillTree v1.3.9' (NullReferenceException: Object reference not set to an instance of an object.)
  at Oxide.Plugins.SkillTree.OnEntityKill (BaseEntity entity) [0x00069] in <eae737c5a6074f26ab883307336a676a>:0 
  at Oxide.Plugins.SkillTree.DirectCallHook (System.String name, System.Object& ret, System.Object[] args) [0x04708] in <eae737c5a6074f26ab883307336a676a>:0 
  at Oxide.Plugins.CSharpPlugin.InvokeMethod (Oxide.Core.Plugins.HookMethod method, System.Object[] args) [0x00079] in <23ba99f131254889867c71f0bd137b1d>:0 
  at Oxide.Core.Plugins.CSPlugin.OnCallHook (System.String name, System.Object[] args) [0x000d8] in <157a94ee66ab4a7991faecd1eb84be3b>:0 
  at Oxide.Core.Plugins.Plugin.CallHook (System.String hook, System.Object[] args) [0x00060] in <157a94ee66ab4a7991faecd1eb84be3b>:0 

Hi! You should to check if you have this: 
https://umod.org/plugins/spawn-modular-car

yes, I see another ticket and already install it, this error showing after installed.

should I reboot server or something?

You need to reboot the plugin

still getting error message even reboot the server