NullReference

I have gotten a null reference exception on line 10

    using UnityEngine;
     using System.Collections;
     
     public class MainMenu : MonoBehaviour {
     public int ServerPort = 25565;
     public int MaxiPlayers = 27;
     
     void OnGUI(){
     if(GUI.Button(new Rect(Screen.width/2-310,Screen.height/2-175,183,28),"StartServer")){
     NetworkManager2.Instance.StartServer(MaxiPlayers,ServerPort);
     }
     }
   }

And on the script NetworkManager2

 using UnityEngine;
 using System.Collections;
 
 public class NetworkManager2 : MonoBehaviour {
     public static NetworkManager2 Instance;
 
     // Use this for initialization
     void Start () {
         Instance = this;
     }
     public void StartServer(int MaxPlayers, int Port){
         Debug.Log ("Good");
     }
 }
 

I don't know why I get the error! I heard that you can ignore it but then it does not do the void (It does not do what is in StartServer!).