Help with multiplayer chat functionality, null reference exception using networkmanager

Following an example I found online, this code is supposed to give simple chat functionality to a game. I am getting a null reference exception from one line of code and I was hoping someone could help me resolve the issue. It results in nothing being sent in the chat.

This is the line of code causing the issue around 40-45:

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

Help would be greatly appreciated, thanks in advance.

//---------------BEGIN CHAT CODE---------------------//

 public class MyMessages
 {
     public enum MyMessageTypes
     {
         CHAT_MESSAGE = 1000
     }

     public class ChatMessage : MessageBase
     {
         public string message;
     }
 }

 //--------------------Setup Client and Server---------------//

 public override void OnStartClient(NetworkClient mClient)
 {
     base.OnStartClient(mClient);
     mClient.RegisterHandler((short)MyMessages.MyMessageTypes.CHAT_MESSAGE, OnClientChatMessage);
 }

 public override void OnStartServer()
 {
     base.OnStartServer();
     NetworkServer.RegisterHandler((short)MyMessages.MyMessageTypes.CHAT_MESSAGE, OnServerChatMessage);
 }


 //---------------------Chat Code---------------------------//

 public List <string> chatHistory = new List<string> { };

 //keep track of current message
 private string currentMessage = string.Empty;
 

 //SEND MESSAGE CODE
 private void SendMessage()
 {
     if (!string.IsNullOrEmpty(currentMessage.Trim()))
     {
         MyMessages.ChatMessage msg = new MyMessages.ChatMessage();
         msg.message = currentMessage;
         NetworkLobbyManager.singleton.client.Send((short)MyMessages.MyMessageTypes.CHAT_MESSAGE, msg);
         currentMessage = string.Empty;
     }
 }


 // GUI CODE
 private void OnGUI()
 {
     Top();
 }

 private void Top()
 {
     GUILayout.BeginHorizontal(GUILayout.Width(250));
     currentMessage = GUI.TextField(new Rect(0, Screen.height - 20, 200, 20), currentMessage);
     if (GUI.Button(new Rect(200, Screen.height-20,75,20),"Send"))
         SendMessage();
     GUILayout.EndHorizontal();

     foreach (var msg in chatHistory)
         GUILayout.Label(msg);
 }


 //NETWORK CODE
 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);
 }