Object reference not set to an instance of an object.

I keep getting this error whenever I try opening my Unity project in Visual Studios when building it as windows 10 universal app. Here is my code using UnityEngine; using System.Collections;

public class character_movement : MonoBehaviour {

 public float moveSpeed;
 public float jumpHeight;


 public Transform groundCheck;
 public float groundCheckRadius;
 public LayerMask whatIsGround;
 private bool grounded;

 public Transform wallCheckLeft;
 public Transform wallCheckRight;

 public float wallCheckRadiusLeft;
 public float wallCheckRadiusRight;

 public LayerMask whatIsWall;

 private bool wallSliding;
 private bool doubleJump;

 

 void Start () {

 }// start

 void FixedUpdate(){
     grounded = Physics2D.OverlapCircle (groundCheck.position, groundCheckRadius, whatIsGround);
     wallSliding = Physics2D.OverlapCircle (wallCheckLeft.position, wallCheckRadiusLeft, whatIsWall); //check wall left hand
     wallSliding = Physics2D.OverlapCircle (wallCheckRight.position, wallCheckRadiusRight, whatIsWall); //check wall right hand

 }//Fixed Update




 void Update () {

     if (grounded) {
         doubleJump = false;
     }

     if(Input.GetKeyDown(KeyCode.Space)&& grounded){
         GetComponent<Rigidbody2D>().velocity = new Vector2(GetComponent<Rigidbody2D>().velocity.x, jumpHeight);
     }// if jump


     if (wallSliding) {
         doubleJump = false;
         GetComponent<Rigidbody2D>().velocity = new Vector2(GetComponent<Rigidbody2D>().velocity.y, -1);
     }

     if(Input.GetKeyDown(KeyCode.Space)&& wallSliding){
         GetComponent<Rigidbody2D>().velocity = new Vector2(GetComponent<Rigidbody2D>().velocity.x, jumpHeight);
     }// if jump


     if (Input.GetKeyDown (KeyCode.Space) && !doubleJump && !grounded) {
         GetComponent<Rigidbody2D>().velocity = new Vector2(GetComponent<Rigidbody2D>().velocity.x, jumpHeight);
         doubleJump = true;
     }//if  double jump


     if(Input.GetKey(KeyCode.D)){
         GetComponent<Rigidbody2D>().velocity = new Vector2(moveSpeed, GetComponent<Rigidbody2D>().velocity.y);
     }// if press D
     
     if(Input.GetKey(KeyCode.A)){
         GetComponent<Rigidbody2D>().velocity = new Vector2(-moveSpeed, GetComponent<Rigidbody2D>().velocity.y);
     }// if press A

 }// update

} //main

using UnityEngine; using System.Collections;

public class Checkpoint : MonoBehaviour {

 // Use this for initialization
 void Start () {
 
 }
 
 // Update is called once per frame
 void Update () {
 
 }

}

using UnityEngine; using System.Collections;

public class level_manager : MonoBehaviour {

 public GameObject currentCheckpoint;
 private character_movement player;

 // Use this for initialization
 void Start () {
     player = FindObjectOfType<character_movement> ();
 }
 
 // Update is called once per frame
 void Update () {
 
 }

 public void Respawn(){
     Debug.Log("Player Respawn");
     player.transform.position = currentCheckpoint.transform.position;
 }//Respawn

}

using UnityEngine; using System.Collections;

public class player_death : MonoBehaviour {

 public level_manager levelManager;

 void OnTriggerEnter2D(Collider2D other){
     if (other.name == "Player") {
         levelManager.Respawn();
     }//if player
 }//Trigger

}