NullReferenceException in game build

I just built my first person horror game and upon a quick playthrough, when I reach the second level, the camera cannot move up or down but everything else can happen. When I went back to Unity to see what the problem was, it didn't happen, it only happens in the built version of the game. When I bring up the developer console in the build it comes up with NullReferenceException error? I think it's an error.

After searching the unity Answers for a while I tested that the camera in my controller is tagged as main camera and as far as I can tell, there are no other cameras in the scene.

 using UnityEngine;
 using System.Collections;
 
 [RequireComponent (typeof(CharacterController))]
 public class FirstPersonController : MonoBehaviour 
 {
     public float forwardmovementSpeed = 7.0f;
     public float sidemovementSpeed = 5.0f;
     public float jumpSpeed = 0.1f;
     public float mouseXSensitivity;
     public float mouseYSensitivity;
     float verticalRotation = 0;
     public float upDownRange = 60.0f;
     float verticalVelocity = 0;
     CharacterController characterController;
     public float Stamina = 10;
     public bool Run;
     
     void Awake ()
     {
         DontDestroyOnLoad(gameObject);
     }
 
     void Start () 
     {
         mouseXSensitivity = PlayerPrefs.GetFloat("Sensitivity");
         mouseYSensitivity = PlayerPrefs.GetFloat("Sensitivity");
         Screen.lockCursor = true;
         characterController = GetComponent<CharacterController>();
         Camera.main.fieldOfView = PlayerPrefs.GetInt("FOV");
     }
 
     void Update () 
     {
         //ROTATION
         float rotLeftRight = Input.GetAxis("Mouse X") * mouseXSensitivity;
         transform.Rotate (0, rotLeftRight, 0);
         
         verticalRotation -= Input.GetAxis("Mouse Y") * mouseYSensitivity;
         verticalRotation = Mathf.Clamp(verticalRotation, -upDownRange, upDownRange);
         Camera.main.transform.localRotation = Quaternion.Euler(verticalRotation, 0, 0);
         
         
         //MOVEMENT
         float forwardSpeed = Input.GetAxis("Vertical") * forwardmovementSpeed;
         float sideSpeed = Input.GetAxis("Horizontal") * sidemovementSpeed;
         verticalVelocity += Physics.gravity.y * Time.deltaTime;
         
         if(characterController.isGrounded && Input.GetButtonDown("Jump"))
         {
             verticalVelocity = jumpSpeed;    
         }
         
         Vector3 speed = new Vector3(sideSpeed, verticalVelocity, forwardSpeed);
         speed = transform.rotation * speed;
         characterController.Move(speed * Time.deltaTime);
 
         if(Input.GetKeyDown(KeyCode.LeftShift))
         {
             if(Stamina > 0)
             {
                 Run = true;
                 forwardmovementSpeed = 7f;
                 sidemovementSpeed = 5f;
             }
         }
         if(Stamina <= 0)
         {
             forwardmovementSpeed = 3f;
             sidemovementSpeed = 3f;
             Run = false;
         }
 
         if(Input.GetKeyUp(KeyCode.LeftShift))
         {
             forwardmovementSpeed = 3f;
             sidemovementSpeed = 3f;
             Run = false;
         }
 
         if(Run == true)
         {
             Stamina -= Time.deltaTime;
         }
         else
         {
             if(Stamina < 10)
             {
                 Stamina += Time.deltaTime;
             }
         }
         if(Input.GetKeyDown(KeyCode.F1))
         {
             Application.LoadLevel(3);
         }
     }
 }
 

Any help would be appreciated.