Whats wrong with this mutiplayer script

hey i am following the tutorial but i am facing a problem here , the problem is

NullReferenceException: Object reference not set to an instance of an object MainManager.Menu_Main () (at Assets/Scripts/MainManager.cs:47) MainManager.OnGUI () (at Assets/Scripts/MainManager.cs:23)

my both script are

using UnityEngine; using System.Collections;

public class MainManager : MonoBehaviour {

public string CurrentMenu; public string MatchName; public string MatchPassword; public int MatchMaxPlayers = 32 ;

 private Vector2  ScrollLobby = Vector2.zero ;
 

void Start() { CurrentMenu = "Main"; MatchName = "match name" + Random.Range(0,5000); }

void OnGUI() { if(CurrentMenu == "Main") Menu_Main(); if(CurrentMenu == "Lobby") Menu_Lobby(); if(CurrentMenu == "Host") Menu_HostGame();

 }

public void NavigateTo(string nextmenu) { CurrentMenu = nextmenu; //stores the value of current menue
}

private void Menu_Main() { if(GUI.Button(new Rect(10, 20, 200, 50),"Host game")) { NavigateTo("Host"); } if(GUI.Button(new Rect(10, 70, 200, 50),"Refresh")) { MasterServer.RequestHostList("DeathMatch");

     }
 MultiplayerManager.instance.PlayerName = GUI.TextField(new Rect(300, 20, 150, 50),MultiplayerManager.instance.PlayerName);    
 GUI.Label(new Rect(220, 20, 150, 50),"Player Name");
 
   if(GUI.Button(new Rect(460, 10, 100, 30),"Save Name"))
     {
         PlayerPrefs.SetString("PlayerName",MultiplayerManager.instance.PlayerName);
     }
 
 GUILayout.BeginArea(new Rect(Screen.width - 400 ,0,400, Screen.height),"Server List","Box");
     
     foreach(HostData match in MasterServer.PollHostList())
     {
         GUILayout.BeginHorizontal("BOX");
         
         GUILayout.Label(match.gameName);
         if(GUILayout.Button ("Connect"))
         {
             Network.Connect(match);
         }    
         
         
         GUILayout.EndHorizontal();
     }
     
     
 GUILayout.EndArea();    
 }
 
 
 

private void Menu_HostGame() { if(GUI.Button(new Rect(10, 20, 200, 50),"Back")) { NavigateTo("Main"); }
if(GUI.Button(new Rect(10, 70, 200, 50),"Start Server")) { MultiplayerManager.instance.StartServer(MatchName, MatchPassword,MatchMaxPlayers);
} GUI.Label(new Rect(220,10,130,30),"Match Name"); MatchName = GUI.TextField(new Rect(400,10,200,30),MatchName);

     GUI.Label(new Rect(220,40,130,30),"Match Password");
     MatchPassword = GUI.PasswordField(new Rect(400,40,200,30),MatchPassword,'*');
     
     GUI.Label(new Rect(220,70,130,30),"Match Max Players");
     GUI.Label(new Rect(400,70,200,30),MatchMaxPlayers.ToString());
     MatchMaxPlayers = Mathf.Clamp(MatchMaxPlayers ,8,32);
     
     if(GUI.Button(new Rect(425,70,25,30),"+"))
         MatchMaxPlayers +=2;
     if(GUI.Button (new Rect(450,70,25,30),"-"))
         MatchMaxPlayers -=2;
 
 
 
 }    

private void Menu_Lobby() { ScrollLobby = GUILayout.BeginScrollView(ScrollLobby,GUILayout.MaxWidth(200));

     foreach(MPPlayer pl in MultiplayerManager.instance.PlayerList)
     {
         GUILayout.Box(pl.PlayerName);
     }
     
     GUILayout.EndScrollView();
     
     
 }    
 

void OnServerInitialized() { NavigateTo("Lobby"); }

void OnConnectedToServer() { NavigateTo("Lobby"); }

}

and 2nd script MultiplayerManager

using UnityEngine; using System.Collections; using System.Collections.Generic; public class MultiplayerManager : MonoBehaviour {

 public static MultiplayerManager instance ;
 private string MatchName = " ";
 private string MatchPassword = " ";
 private int MatchMaxUsers = 32;
 public string PlayerName = "Ali"  ;
 
 public List< MPPlayer> PlayerList = new List(); 
 
 
 void start()
 {
     instance = this;
     PlayerName = PlayerPrefs.GetString ("PlayerName");
 }
 
 public void StartServer(string servername , string serverpassword, int maxusers)
 {
     MatchName = servername;
     MatchPassword = serverpassword;
     MatchMaxUsers = maxusers;
     Network.InitializeServer(MatchMaxUsers,2550,false);
     MasterServer.RegisterHost("Death Match",MatchName,"");
     //Network.InitializeSecurity();
     
 }
 void OnServerInitialized()
 {
     Server_PlayerJoinRequest(PlayerName,Network.player);
 }
 
 void OnConnectedToServer()
 {
     networkView.RPC("Server_PlayerJoinRequest",RPCMode.Server,PlayerName,Network.player);
 }
 void OnPlayerDisconnected(NetworkPlayer id)
 {
     networkView.RPC("Client_RemovePlayer",RPCMode.All, id);
 }
 [RPC]
 void Server_PlayerJoinRequest(string playername, NetworkPlayer view)
 {
     networkView.RPC("client_AddPlayerToList",RPCMode.All, playername, view);
 }
 [RPC]
 void client_AddPlayerToList(string playername, NetworkPlayer view)
 {
     MPPlayer tempplayer = new MPPlayer();
     tempplayer.PlayerName = playername;
     tempplayer.PlayerNetwork = view;
     PlayerList.Add(tempplayer);
 }
 [RPC]
 void Client_RemovePlayer(NetworkPlayer view)
 {
     MPPlayer  temppl = null;
     foreach(MPPlayer pl in PlayerList)
     {
         if(pl.PlayerNetwork == view)
         {
             temppl = pl;
         }
     }
     if(temppl != null)
     {
         PlayerList.Remove(temppl);
     }
     
 }

}

public class MPPlayer { public string PlayerName= ""; public NetworkPlayer PlayerNetwork; }