I have an error saying its not set to an instance of an object althoug i have set it to an object. Please Help ??

This is the error :
NullReferenceException: Object reference not set to an instance of an object MenuManager.Menu_Main () (at Assets/MyScripts/MenuManager.cs:40) MenuManager.OnGUI () (at Assets/MyScripts/MenuManager.cs:20)

 using UnityEngine;
 using System.Collections;
 
 public class MenuManager : MonoBehaviour 
 {
     public string CurrentMenu;
 
     public string MatchName;
     public string MatchPassword;
     public int MatchMaxPlayers = 32;
 
     void start()
     {
         CurrentMenu = "Main";
     }
 
     void OnGUI()
     {
         if (CurrentMenu == "Main")
             Menu_Main ();
         if (CurrentMenu == "Lobby")
             Menu_Lobby ();
         if (CurrentMenu == "Host")
             Menu_HostGame ();
     }
 
     public void NavigateTo(string nextmenu)
     {
         CurrentMenu = nextmenu;
     }
 
     private void Menu_Main()
     {
         if(GUI.Button(new Rect(10, 10, 200, 50), "Host Game"))
         {
             NavigateTo("Host");
         }
 
         GUI.Label (new Rect (220, 10, 130, 30), "Player Name");
         MultiplayerManager.instance.PlayerName = GUI.TextField (new Rect (325, 22, 200, 30), MultiplayerManager.instance.PlayerName);
 
     }
 
     private void Menu_HostGame()
     {
         //Buttons Host Game
         if (GUI.Button(new Rect(10, 10, 200, 50), "Back"))
         {
             NavigateTo("Main");
         }
 
         if (GUI.Button (new Rect (10, 60, 200, 50), "Start Server")) 
         {
             MultiplayerManager.instance.StartServer(MatchName, MatchPassword, MatchMaxPlayers);
         }
         GUI.Label (new Rect (220, 26, 200, 20), "Match Name");
         MatchName = GUI.TextField (new Rect (325, 22, 200, 30), MatchName);
 
         GUI.Label (new Rect (220, 66, 200, 20), "Match Password");
         MatchPassword = GUI.PasswordField (new Rect (325, 62, 200, 30), MatchPassword, "*"[0], 25);
 
         GUI.Label (new Rect (220, 106, 200, 20), "Max Players");
         GUI.Label (new Rect (320, 107, 200, 30), MatchMaxPlayers.ToString());
         MatchMaxPlayers = Mathf.Clamp (MatchMaxPlayers, 8, 32);
 
         if (GUI.Button (new Rect (370, 104, 20, 20), "+"))
             MatchMaxPlayers += 1;
         if (GUI.Button (new Rect (340, 104, 20, 20), "-"))
             MatchMaxPlayers -= 1;
 
     }
 
     private void Menu_Lobby()
     {
 
     }
 
 }

 using UnityEngine;
 using System.Collections;
 using System.Collections.Generic;
 
 
 public class MultiplayerManager : MonoBehaviour
 {
     public static MultiplayerManager instance;
     
     
     public string PlayerName;
     
     private string MatchName = "";
     private string MatchPassword = "";
     private int MatchMaxUsers = 32;
 
     public List<MPPlayer> PlayerList = new List<MPPlayer>();
 
     void start()
     {
         instance = this;
     }
 
     public void StartServer(string servername, string serverpassword, int maxplayers)
     {
         MatchName = servername;
         MatchPassword = serverpassword;
         MatchMaxUsers = maxplayers;
         Network.InitializeServer (MatchMaxUsers, 2550, false);
         Network.InitializeSecurity ();
     }
 
     void OnServerInitialized()
     {
         Server_PlayerJoinRequest ("", Network.player);
     }
 
     void OnConnectedToServer()
     {
         GetComponent<NetworkView>().RPC("Server_PlayerJoinRequest", RPCMode.Server, "", Network.player);
     }
 
     void OnPlayerDisconnected(NetworkPlayer id)
     {
         GetComponent<NetworkView>().RPC("Client_RemovePlayer", RPCMode.All, id);
     }
 
     [RPC]
     void Server_PlayerJoinRequest(string playername, NetworkPlayer view)
     {
         GetComponent<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;
 }