Getting error when making a chat system

This is my error:

    NullReferenceException: Object reference not set to an instance of an object
     NetworkManager.AddChatMessage (System.String m) (at Assets/Scripts/Network/NetworkManager.cs:30)

From this tutorial

This is the code the error is coming from:

 using UnityEngine;
 using System.Collections.Generic;
     
 public class NetworkManager : MonoBehaviour {
     
     public GameObject standbyCamera;
     SpawnSpot[] spawnSpots;
 
     bool connecting = false;
 
     List<string> chatMessages;
 
     int maxChatMessages = 5;
 
     // Use this for initialization
     void Start () {
         spawnSpots = GameObject.FindObjectsOfType<SpawnSpot>();
         PhotonNetwork.player.name = PlayerPrefs.GetString("Username", "Player");
         chatMessages = new List<string>();
     }
 
     void OnDestroy() {
         PlayerPrefs.GetString ("Username", PhotonNetwork.player.name);
         //Hashtable props = new Hashtable ();
         //props["asdasda"] = 2;
         //PhotonNetwork.player.SetCustomProperties(props);
     }
     
     public void AddChatMessage(string m) {
         GetComponent<PhotonView>().RPC ("AddChatMessage_RPC", PhotonTargets.All, m);
     }
     
     [RPC]
     void AddChatMessage_RPC(string m) {
         while(chatMessages.Count >= maxChatMessages) {
             chatMessages.RemoveAt(0);
         }
         chatMessages.Add(m);
     }
 
     void Connect() {
         PhotonNetwork.ConnectUsingSettings( "Alpha 0.2" );
 
     }
 
     
     void OnGUI() {
         GUILayout.Label( PhotonNetwork.connectionStateDetailed.ToString() );
         GUILayout.BeginArea (new Rect(0, 0, Screen.width, Screen.height) );
         GUILayout.BeginHorizontal ();
         GUILayout.FlexibleSpace ();
         GUILayout.BeginVertical ();
         GUILayout.FlexibleSpace ();
         
 
         GUILayout.BeginHorizontal ();
         GUILayout.Label ("Username");
         PhotonNetwork.player.name = GUILayout.TextField (PhotonNetwork.player.name);
         GUILayout.EndHorizontal ();
 
         if (PhotonNetwork.connected == false && connecting == false ) {
             if(GUILayout.Button("Single Player")){
                 connecting = true;
                 PhotonNetwork.offlineMode = true;
                 OnJoinedLobby();
             }
             if(GUILayout.Button("Multi Player")){
                 connecting = true;
                 Connect();
             }
             GUILayout.FlexibleSpace ();
             GUILayout.EndVertical ();
             GUILayout.FlexibleSpace ();
             GUILayout.EndHorizontal();
             GUILayout.EndArea();
         }
         if (PhotonNetwork.connected == true && connecting == false) {
             GUILayout.BeginArea (new Rect(0, 0, Screen.width, Screen.height) );
             GUILayout.BeginHorizontal ();
             GUILayout.FlexibleSpace ();
 
             foreach(string msg in chatMessages){
                 GUILayout.Label(msg);
             }
 
             GUILayout.EndVertical ();
             GUILayout.EndArea();
             
         }
     }
     
     void OnJoinedLobby() {
         Debug.Log ("OnJoinedLobby");
         PhotonNetwork.JoinRandomRoom();
     }
     
     void OnPhotonRandomJoinFailed() {
         Debug.Log ("OnPhotonRandomJoinFailed");
         PhotonNetwork.CreateRoom( null );
     }
     
     void OnJoinedRoom() {
         Debug.Log ("OnJoinedRoom");
             connecting = false;
         SpawnMyPlayer();
     }
     
     void SpawnMyPlayer() {
         AddChatMessage ("Spawning player: " + PhotonNetwork.player.name);
 
         if(spawnSpots == null) {
             Debug.LogError ("WTF?!?!?");
             return;
         }
         
         SpawnSpot mySpawnSpot = spawnSpots[ Random.Range (0, spawnSpots.Length) ];
         GameObject myPlayerGO = (GameObject)PhotonNetwork.Instantiate("Player", mySpawnSpot.transform.position, mySpawnSpot.transform.rotation, 0);
         standbyCamera.SetActive(false);
         
         //((MonoBehaviour)myPlayerGO.GetComponent("FPSInputController")).enabled = true;
         ((MonoBehaviour)myPlayerGO.GetComponent("MouseLook")).enabled = true;
         ((MonoBehaviour)myPlayerGO.GetComponent("Crosshair")).enabled = true;
         ((MonoBehaviour)myPlayerGO.GetComponent("Pause1")).enabled = true;
         ((MonoBehaviour)myPlayerGO.GetComponent("PlayerMovement")).enabled = true;
         myPlayerGO.transform.FindChild("Main Camera").gameObject.SetActive(true);
     }
 }