How do i fix this error?,I Have A Problem With The Destroy Command

when i have my game started and i got hit, there is an error that says:"The object of Type 'GameObject' has been destroyed but you are still trying to acces it. Your script should either check if it is null or you shold not destroy the Object"

here is my script that should destroy the Player, :

using System.Collections; using UnityEngine.SceneManagement; using System.Collections.Generic; using UnityEngine; public class Move2d : MonoBehaviour {

 public GameObject deadexplosion;
 public float speed;
 public float heavy;

 private Rigidbody2D rb2d;

 void Start()
 {
     rb2d = GetComponent<Rigidbody2D>();
 }
 void FixedUpdate()
 {
     float moveHorizontal = Input.GetAxis("Horizontal");
     float moveVertical = Input.GetAxis("Vertical");
     Vector2 movement = new Vector2(moveHorizontal, moveVertical);
     rb2d.AddForce(movement * speed / heavy);
 }

 void dead()
 {
     Destroy(GameObject.FindGameObjectWithTag("Player"));
 }

 void BackToMainMenu()
 {
     SceneManager.LoadScene("Mainmenu");
 }

 private void OnTriggerEnter2D(Collider2D collision)
 {
     if (collision.tag == "Enemy")
         dead();
     BackToMainMenu();
 }

};

And here is the script that has the error, the eroors is "Vector3 difference = target - Player.transform.position :

using UnityEngine;

public class PointAndShoot : MonoBehaviour { private Rigidbody2D rb2d;

 public GameObject crosshairs;

 public GameObject player;

 public GameObject bulletPrefab;

 public GameObject bulletStart;

 public float bulletprefab;

 public float bulletSpeed = 60.0f;

 private Vector3 target;
 void Start()
 {
     Cursor.visible = false;
 }
 private void Update()
 { 

     target = Camera.main.ScreenToWorldPoint(new Vector3(Input.mousePosition.x, Input.mousePosition.y, transform.position.z));
     crosshairs.transform.position = new Vector2(target.x, target.y);

     Vector3 difference = target - player.transform.position;
     float rotationZ = Mathf.Atan2(difference.y, difference.x) * Mathf.Rad2Deg;
     player.transform.rotation = Quaternion.Euler(0.0f, 0.0f, rotationZ - 90.0f);

     if (Input.GetMouseButtonDown(0))
     {
         float distance = difference.magnitude;
         Vector2 direction = difference / distance;
         direction.Normalize();
         fireBullet(direction, rotationZ);
     }
 }
 void fireBullet(Vector2 direction, float rotationZ)
 {
     GameObject b = Instantiate(bulletPrefab) as GameObject;
     b.transform.position = bulletStart.transform.position;
     b.transform.rotation = Quaternion.Euler(0.0f, 0.0f, rotationZ);
     b.GetComponent<Rigidbody2D>().velocity = direction * bulletSpeed;
 }

} I am realy noob and i am happy for every Response :) ,when i have my game started and i got hit, there is an error that says:"The object of Type 'GameObject' has been destroyed but you are still trying to acces it. Your script should either check if it is null or you shold not destroy the Object"

here is my script that should destroy the Player, :

using System.Collections; using UnityEngine.SceneManagement; using System.Collections.Generic; using UnityEngine; public class Move2d : MonoBehaviour {

 public GameObject deadexplosion;
 public float speed;
 public float heavy;

 private Rigidbody2D rb2d;

 void Start()
 {
     rb2d = GetComponent<Rigidbody2D>();
 }
 void FixedUpdate()
 {
     float moveHorizontal = Input.GetAxis("Horizontal");
     float moveVertical = Input.GetAxis("Vertical");
     Vector2 movement = new Vector2(moveHorizontal, moveVertical);
     rb2d.AddForce(movement * speed / heavy);
 }

 void dead()
 {
     Destroy(GameObject.FindGameObjectWithTag("Player"));
 }

 void BackToMainMenu()
 {
     SceneManager.LoadScene("Mainmenu");
 }

 private void OnTriggerEnter2D(Collider2D collision)
 {
     if (collision.tag == "Enemy")
         dead();
     BackToMainMenu();
 }

};

And here is the script that has the error, the eroors is "Vector3 difference = target - Player.transform.position :

using UnityEngine;

public class PointAndShoot : MonoBehaviour { private Rigidbody2D rb2d;

 public GameObject crosshairs;

 public GameObject player;

 public GameObject bulletPrefab;

 public GameObject bulletStart;

 public float bulletprefab;

 public float bulletSpeed = 60.0f;

 private Vector3 target;
 void Start()
 {
     Cursor.visible = false;
 }
 private void Update()
 { 

     target = Camera.main.ScreenToWorldPoint(new Vector3(Input.mousePosition.x, Input.mousePosition.y, transform.position.z));
     crosshairs.transform.position = new Vector2(target.x, target.y);

     Vector3 difference = target - player.transform.position;
     float rotationZ = Mathf.Atan2(difference.y, difference.x) * Mathf.Rad2Deg;
     player.transform.rotation = Quaternion.Euler(0.0f, 0.0f, rotationZ - 90.0f);

     if (Input.GetMouseButtonDown(0))
     {
         float distance = difference.magnitude;
         Vector2 direction = difference / distance;
         direction.Normalize();
         fireBullet(direction, rotationZ);
     }
 }
 void fireBullet(Vector2 direction, float rotationZ)
 {
     GameObject b = Instantiate(bulletPrefab) as GameObject;
     b.transform.position = bulletStart.transform.position;
     b.transform.rotation = Quaternion.Euler(0.0f, 0.0f, rotationZ);
     b.GetComponent<Rigidbody2D>().velocity = direction * bulletSpeed;
 }

} I am realy noob and i am happy for every Response :)