Chat system is not working

Hi everybody,

I've used this code but It is not working so anyone help me?

It broken in this line and it shows nullobject:

 NetworkManager.singleton.client.Send((short) MyMessages.MyMessageTypes.CHAT_MESSAGE, msg);

Here is my code:

 using UnityEngine;
 using System.Collections.Generic;
 using UnityEngine.Networking;
 using UnityEngine.Networking.NetworkSystem;
  
 public class Chat : NetworkManager {
     public class MyMessages
     {
        
         public enum MyMessageTypes
         {
             CHAT_MESSAGE = 1000
         }
        
         public class ChatMessage : MessageBase
         {
             public string message;
         }
        
     }
  
     public List<string> chatHistory = new List<string>();
    
     //Keeps track of current message
     private string currentMessage = string.Empty;
  
     // hook into NetworkManager client setup process
     public override void OnStartClient(NetworkClient mClient)
     {
         base.OnStartClient(mClient); // base implementation is currently empty
        
         mClient.RegisterHandler((short)MyMessages.MyMessageTypes.CHAT_MESSAGE, OnClientChatMessage);
     }
    
    
    
     // hook into NetManagers server setup process
     public override void OnStartServer()
     {
         base.OnStartServer(); //base is empty
         NetworkServer.RegisterHandler((short)MyMessages.MyMessageTypes.CHAT_MESSAGE, OnServerChatMessage);
     }
  
     private void OnGUI() {
         GUILayout.BeginHorizontal(GUILayout.Width(250));
        
         currentMessage = GUILayout.TextField(currentMessage);
  
         if (GUILayout.Button("Send"))
         {
             if (!string.IsNullOrEmpty(currentMessage))
             {
                 Debug.Log("a: "+ currentMessage);
                 MyMessages.ChatMessage msg = new MyMessages.ChatMessage();
                 msg.message = currentMessage;
 //                this.client.Send((short) MyMessages.MyMessageTypes.CHAT_MESSAGE, msg);
                 NetworkManager.singleton.client.Send((short) MyMessages.MyMessageTypes.CHAT_MESSAGE, msg);
                
                 currentMessage = string.Empty;
             }
         }
         foreach (var msg in chatHistory)
         {
             GUILayout.Label(msg);
         }
        
         GUILayout.EndHorizontal();
     }
  
     private void OnServerChatMessage(NetworkMessage netMsg)
     {
         var msg = netMsg.ReadMessage<StringMessage>();
         Debug.Log("New chat message on server: " + msg.value);
        
         MyMessages.ChatMessage chat = new MyMessages.ChatMessage();
         chat.message = msg.value;
        
         if (msg.value == "Dam this game sucks")
         {
             return;
         }
         else
         {
             NetworkServer.SendToAll((short)MyMessages.MyMessageTypes.CHAT_MESSAGE, chat);
         }
     }
  
     private void OnClientChatMessage(NetworkMessage netMsg)
     {
         var msg = netMsg.ReadMessage<StringMessage>();
         Debug.Log("New chat message on client: " + msg.value);
        
         chatHistory.Add(msg.value);
     }
  
 }

Thanks.