Compilation error: (ArgumentNullException: Argument cannot be null. Parameter name: obj)

Code:

[Oxide] 7:52 PM [Error] Compilation error:  (ArgumentNullException: Argument cannot be null.
Parameter name: obj)
[Oxide] 7:52 PM [Debug]  at System.Runtime.Serialization.ObjectIDGenerator.GetId (System.Object obj, System.Boolean& firstTime) [0x00000] in <filename unknown>:0
  at System.Runtime.Serialization.Formatters.Binary.ObjectWriter.WriteObjectInstance (System.IO.BinaryWriter writer, System.Object obj, Boolean isValueObject) [0x00000] in <filename unknown>:0
  at System.Runtime.Serialization.Formatters.Binary.ObjectWriter.WriteQueuedObjects (System.IO.BinaryWriter writer) [0x00000] in <filename unknown>:0
  at System.Runtime.Serialization.Formatters.Binary.ObjectWriter.WriteObjectGraph (System.IO.BinaryWriter writer, System.Object obj, System.Runtime.Remoting.Messaging.Header[] headers) [0x00000] in <filename unknown>:0
  at System.Runtime.Serialization.Formatters.Binary.BinaryFormatter.Serialize (System.IO.Stream serializationStream, System.Object graph, System.Runtime.Remoting.Messaging.Header[] headers) [0x00000] in <filename unknown>:0
  at System.Runtime.Serialization.Formatters.Binary.BinaryFormatter.Serialize (System.IO.Stream serializationStream, System.Object graph) [0x00000] in <filename unknown>:0
  at ObjectStream.IO.ObjectStreamWrapper`2[ObjectStream.Data.CompilerMessage,ObjectStream.Data.CompilerMessage].Serialize (ObjectStream.Data.CompilerMessage obj) [0x00000] in <filename unknown>:0
  at ObjectStream.IO.ObjectStreamWrapper`2[ObjectStream.Data.CompilerMessage,ObjectStream.Data.CompilerMessage].WriteObject (ObjectStream.Data.CompilerMessage obj) [0x00000] in <filename unknown>:0
  at ObjectStream.ObjectStreamConnection`2[ObjectStream.Data.CompilerMessage,ObjectStream.Data.CompilerMessage].WriteStream () [0x00000] in <filename unknown>:0
  at ObjectStream.Threading.Worker.DoWorkImpl (System.Object oAction) [0x00000] in <filename unknown>:0 > reload EventManager
[Oxide] 7:52 PM [Warning] Ignored obsolete game extension reference 'Oxide.Ext.Rust' in plugin 'EventManager'
> reload EventManager
[Oxide] 7:53 PM [Debug] Reload requested for plugin which is already reloading: EventManager

Whats wrong with this? After server restart it worked fine, but i'm not sure why this happens in first place.
ps i'm just tried to fix one issue with this plugin, but this happens and i decided to write here.