Failed to call hook 'OnEntityDeath' on plugin 'NPCDropGun v2.0.1' (NullReferenceException: Object reference not set to an instance of an object)
  at Oxide.Plugins.NPCDropGun.GetRandomSkin (ItemDefinition idef) [0x0001f] in :0
  at Oxide.Plugins.NPCDropGun.DoSpawns (BasePlayer player) [0x000ad] in :0
  at Oxide.Plugins.NPCDropGun.OnEntityDeath (BasePlayer player) [0x00016] in :0
  at Oxide.Plugins.NPCDropGun.DirectCallHook (System.String name, System.Object& ret, System.Object[] args) [0x000b0] in :0
  at Oxide.Plugins.CSharpPlugin.InvokeMethod (Oxide.Core.Plugins.HookMethod method, System.Object[] args) [0x00079] in <33f7c42a56934dae8ec04b11017dce31>:0
  at Oxide.Core.Plugins.CSPlugin.OnCallHook (System.String name, System.Object[] args) [0x000d8] in <33287985591c40d6983f0b8f0e915565>:0
  at Oxide.Core.Plugins.Plugin.CallHook (System.String hook, System.Object[] args) [0x00060] in <33287985591c40d6983f0b8f0e915565>:0

I get this error and they dont drop guns also I dont have settings to set the droprate chance ...

Fixed in the latest update (2.0.2)