NullReferenceException: Object reference not set to an instance of an object NetworkManager.SpawnMyPlayer () (at Assets/NetworkManager.cs:46)

using UnityEngine;
using System.Collections;

public class NetworkManager : MonoBehaviour {
	
	public Camera standbyCamera;
	SpawnSpot[] spawnSpots;

	// Use this for initialization
	void Start () {
		spawnSpots = GameObject.FindObjectsOfType<SpawnSpot>();
		Connect ();	
	}
	
	void Connect() {
		PhotonNetwork.ConnectUsingSettings( "MultiPlayer v001" );	
	}
	
	void OnGUI() {
		GUILayout.Label( PhotonNetwork.connectionStateDetailed.ToString() );
	}
	
	void OnJoinedLobby(){
		PhotonNetwork.JoinRandomRoom();
	}
	
	void OnPhotonRandomJoinFailed(){
		PhotonNetwork.CreateRoom( null );
	}
	
	void OnJoinedRoom(){
		SpawnMyPlayer();
	}
	
	void SpawnMyPlayer(){
		if(spawnSpots == null){
			Debug.LogError ("QUE MIERDA!!!");
			return;
		}
		
		
		SpawnSpot mySpawnSpot = spawnSpots[ Random.Range (0, spawnSpots.Length)];
		GameObject myPlayerGO = (GameObject)PhotonNetwork.Instantiate("Player Controller", mySpawnSpot.transform.position, mySpawnSpot.transform.rotation, 0);
		standbyCamera.enabled = false;
		((MonoBehaviour)myPlayerGO.GetComponent("FPSInputController")).enabled = true;
		((MonoBehaviour)myPlayerGO.GetComponent("MouseLook")).enabled = true;
		myPlayerGO.transform.FindChild("Main Camera").gameObject.SetActive(true);
	}
}

error: NullReferenceException: Object reference not set to an instance of an object
NetworkManager.SpawnMyPlayer () (at Assets/NetworkManager.cs:46)
NetworkManager.OnJoinedRoom () (at Assets/NetworkManager.cs:32)
UnityEngine.GameObject:SendMessage(String, Object, SendMessageOptions)
NetworkingPeer:SendMonoMessage(PhotonNetworkingMessage, Object) (at Assets/Photon Unity Networking/Plugins/PhotonNetwork/NetworkingPeer.cs:1865)
NetworkingPeer:OnEvent(EventData) (at Assets/Photon Unity Networking/Plugins/PhotonNetwork/NetworkingPeer.cs:1708)
ExitGames.Client.Photon.PeerBase:DeserializeMessageAndCallback(Byte)
ExitGames.Client.Photon.EnetPeer:DispatchIncomingCommands()
ExitGames.Client.Photon.PhotonPeer:DispatchIncomingCommands()
PhotonHandler:Update() (at Assets/Photon Unity Networking/Plugins/PhotonNetwork/PhotonHandler.cs:83)

Do yourself a favor and debug your code. It appears it can’t find the MouseLook script, adjust your code to output to your console.

    SpawnSpot mySpawnSpot = spawnSpots[ Random.Range (0, spawnSpots.Length)];
    GameObject myPlayerGO = (GameObject)PhotonNetwork.Instantiate("Player Controller", mySpawnSpot.transform.position, mySpawnSpot.transform.rotation, 0);
    standbyCamera.enabled = false;
    
    MonoBehaviour fpsInput = myPlayerGO.GetComponent("FPSInputController") as MonoBehaviour;
	if (fpsInput != null)
	{
      fpsInput.enabled = true;
	  Debug.Log("FPSInputController is enabled");
	}
    else
	{
      Debug.Log("Couldn't find a reference to FPSInputController on myPlayerGO");
	}
	//((MonoBehaviour)myPlayerGO.GetComponent("FPSInputController")).enabled = true;
    
	MonoBehaviour mouseLook = myPlayerGO.GetComponent("MouseLook") as MonoBehaviour;
    if(mouseLook != null)
	{
      mouseLook.enabled = true;
	  Debug.Log("MouseLook is enabled");
    }
    else
	{
      Debug.Log("Couldn't find a reference to MouseLook on myPlayerGO");
	}
			
    //((MonoBehaviour)myPlayerGO.GetComponent("MouseLook")).enabled = true;
    myPlayerGO.transform.FindChild("Main Camera").gameObject.SetActive(true);

It means you have some error in the NetworkManager script, before line 66. Check for missing parenthesis, brackets, semicolon, things like that.