Please help me with this error

Hello to everyone.My goal is to build a 2D platform game.I have this code:

 using System.Collections;
 using System.Collections.Generic;
 using UnityEngine;
 
 public class Camerafollow2DPlatformer : MonoBehaviour {
 
     public Transform target;
     public float smoothing;
 
     Vector3 offset;
     float lowY;
 
     // Use this for initialization
     void Start () {
         offset = transform.position - target.position;
         lowY = transform.position.y;
     }
     
     // Update is called once per frame
     void FixedUpdate () {
         Vector3 targetCamPos = target.position + offset;
         transform.position = Vector3.Lerp (transform.position, targetCamPos, smoothing * Time.deltaTime);
         if (transform.position.y < lowY) transform.position = new Vector3 (transform.position.x, lowY, transform.position.z);
     }
 }
 

I am getting this error: MissingReferenceException: The object of type 'Transform' has been destroyed but you are still trying to access it. Your script should either check if it is null or you should not destroy the object. UnityEngine.Transform.get_position () (at C:/buildslave/unity/build/artifacts/generated/common/runtime/TransformBindings.gen.cs:26) Camerafollow2DPlatformer.FixedUpdate () (at Assets/Scripts/Camerafollow2DPlatformer.cs:21)

The problem is in line 21 but i can't fix it.Please help me.

No my playerScript

 using System.Collections;
 using System.Collections.Generic;
 using UnityEngine;
 
 public class PlayerController : MonoBehaviour {
 
     public float maxSpeed;
     public Vector3 respawnPoint;
 
     bool grounded = false;
     float groundCheckRadius = 0.2f;
     public LayerMask groundLayer;
     public Transform groundCheck;
     public float jumpHeight;
 
     Rigidbody2D myRB;
     Animator myAnim;
     bool facingRight;
 
     public Transform gunTip;
     public GameObject bullet;
     float fireRate = 0.5f;
     float nextFire = 0f;
 
     // Use this for initialization
     void Start () {
         myRB = GetComponent<Rigidbody2D> ();
         myAnim = GetComponent<Animator> ();
         respawnPoint = transform.position;
 
         facingRight = true;
 
     }
     
     // Update is called once per frame
 
     void Update(){
         if (grounded && Input.GetAxis ("Jump") > 0) {
             grounded = false;
             myAnim.SetBool ("isGrounded", grounded);
             myRB.AddForce (new Vector2 (0, jumpHeight));
         }
 
         if (Input.GetAxisRaw ("Fire1") > 0) fireRocket ();
 
     }
 
 
 
     void FixedUpdate () {
 
         grounded = Physics2D.OverlapCircle (groundCheck.position, groundCheckRadius, groundLayer);
         myAnim.SetBool ("isGrounded", grounded);
         myAnim.SetFloat ("VerticalSpeed", myRB.velocity.y);
 
         float move = Input.GetAxis ("Horizontal");
         myAnim.SetFloat ("speed", Mathf.Abs (move));
         myRB.velocity = new Vector2 (move * maxSpeed, myRB.velocity.y);
 
         if (move > 0 && !facingRight) {
             flip ();
         } else if (move < 0 &&facingRight) {
             flip ();
      }
 }
     void flip(){
         facingRight = !facingRight;
         Vector3 theScale = transform.localScale;
         theScale.x *=-1;
         transform.localScale = theScale;  
     }
 
     void fireRocket(){
         if (Time.time > nextFire) {
             nextFire = Time.time + fireRate;
             if (facingRight) {
                 Instantiate (bullet, gunTip.position, Quaternion.Euler (new Vector3 (0, 0, 0)));
             } else if (!facingRight) {
                 Instantiate (bullet, gunTip.position, Quaternion.Euler (new Vector3 (0, 0, 180f)));
             }
         }
     }
 
     void OnTriggerEnter2D(Collider2D other){
         if (other.tag == "cleaner") {
             transform.position = respawnPoint;
         }
         if (other.tag == "Checkpoint") {
             respawnPoint = other.transform.position;
         }
     }        
 }
 

My playerHealth Script

 using System.Collections;
 using System.Collections.Generic;
 using UnityEngine;
 using UnityEngine.UI;
 
 public class PlayerHealth : MonoBehaviour {
 
     public AudioClip PlayerDeathSound;
     public float fullHealth;
     public GameObject deathFX;
     public AudioClip playerHurt;
 
     public restartGame theGameManager;
 
     float currentHealth;
     PlayerController controlMovement;
 
     AudioSource playerAS;
 
     public Slider healthSlider;
     public Image damageScreen;
     public Text gameOverScreen;
     public Text winGameScreen;
 
     bool damaged = false;
     Color damagedColour = new Color(0f,0f,0f,0.5f);
     float smoothColour = 5f;
 
 
     // Use this for initialization
     void Start () {
         currentHealth = fullHealth;
         controlMovement = GetComponent<PlayerController> ();
 
         healthSlider.maxValue = fullHealth;
         healthSlider.value = fullHealth;
 
         damaged = false;
 
         playerAS = GetComponent<AudioSource> ();
     }
     
     // Update is called once per frame
     void Update () {
         
         if (damaged) {
             damageScreen.color = damagedColour;
         } else {
             damageScreen.color = Color.Lerp (damageScreen.color, Color.clear, smoothColour * Time.deltaTime);
         }
         damaged = false;
    }
 
     public void addDamage(float damage){
         if (damage <= 0) return;
         currentHealth -= damage;
         playerAS.clip = playerHurt;
         playerAS.Play ();
         //playerAS.PlayOneShot (playerHurt);
 
 
         healthSlider.value = currentHealth;
         damaged = true;
 
         if (currentHealth<=0){
             makeDead ();
      }
 }
 
     public void addHealth(float healthAmount){
         currentHealth += healthAmount;
         if (currentHealth > fullHealth) currentHealth=fullHealth;
         healthSlider.value = currentHealth;
     }
 
     public void makeDead (){
     Instantiate(deathFX, transform.position, transform.rotation);
     Destroy(gameObject);
     AudioSource.PlayClipAtPoint (PlayerDeathSound, transform.position);
     damageScreen.color = damagedColour;
 
         Animator gameOverAnimator = gameOverScreen.GetComponent<Animator> ();
         gameOverAnimator.SetTrigger ("gameOver");
   }
 
     public void winGame(){
         Destroy (gameObject);
         //theGameManager.restartTheGame ();
         //Animator winGameAnimator = winGameScreen.GetComponent<Animator> ();
         //winGameAnimator.SetTrigger ("gameOver");
     }
 }