Third Party ReplacePlayerForConnection Mirror

Hi all,
I have problems executing the ReplacePlayerForConnection function, I get the error: NullReferenceException: Object reference not set to an instance of an object on the line: GameObject oldPlayer = conn.identity.gameObject;

I'm trying to replace the player's gameobject based on the gamemode: the entire code is:

Code (CSharp):

1. using System.Collections;
2. using System.Collections.Generic;
3. using UnityEngine;
4. using Mirror;
5. using UnityEngine.SceneManagement;
6.  
7. public class ClassController : NetworkManager
8. {
9.     public NetworkConnectionToClient connectionToClient;
10.     public GameObject playerRun;
11.     //
12.  
13.     private void Start()
14.     {
15.  
16.     }
17.     private void Update()
18.     {
19.         SceneManager.sceneLoaded += OnSceneLoaded;
20.     }
21.     public void OnSceneLoaded(Scene Run, LoadSceneMode Single)
22.     {
23.  
24.         ReplacePlayer(connectionToClient, playerRun);
25.  
26.     }
27.     public void ReplacePlayer(NetworkConnectionToClient conn, GameObject newPrefab)
28.     {
29.         // Cache a reference to the current player object
30.         GameObject oldPlayer = conn.identity.gameObject;
31.  
32.         // Instantiate the new player object and broadcast to clients
33.         // Include true for keepAuthority paramater to prevent ownership change
34.         NetworkServer.ReplacePlayerForConnection(conn, Instantiate(newPrefab), true);
35.  
36.         // Remove the previous player object that's now been replaced
37.         // Delay is required to allow replacement to complete.
38.         Destroy(oldPlayer, 0.1f);
39.     }
40.  
41.  
42. }

What I can be doing wrong?