"Transform" still trying to access after Destroy

Hello everyone, I'm making a game where the character when "Right Clik" aiming to a target to shoot it. But after destroy the gameobject, when I'm aiming again it say "Transform" still trying to access after Destroy. I don't know what to do :( Here's my code part of my code :

 #pragma strict
 
 var distance : int;
 var lookAtDistance = 10;
 var target : Transform;
 var damping = 6.0;
 var fireSpeed : float = 15;
 private var waitTillNextFire : float = 0;
 var bullet : GameObject;
 var bulletSpawn : GameObject;
 var bulletSound : GameObject;
 var muzzleFlash : GameObject;
 
 var reloading : boolean = false;
 var reloadingAnim : Animation;
 var reloadingAnimString : String;
 var reloadingSound : AudioSource;
 
 var magSize : int = 15;
 var currentMag : int = 15;
 var maxExtraAmmo : int = 250;
 var currentExtraAmmo : int = 250;
 
 var bulletHudTexture : Texture;
 var ammoCountRect : Rect = Rect(25,25,50,25);
 var ammoStartX : int = 100;
 var ammoY : float = 25;
 var ammoSize : Vector2 = Vector2(10,31);
 var ammoSpacing : float = 4;
 
 function Update () 
 {
 
     distance = Vector3.Distance(target.position, transform.position);
 
     if(Input.GetKey(KeyCode.Mouse1) && distance < lookAtDistance)
     {
         LookAt();
         GameObject.Find("player").GetComponent(playerMovement).enabled = false;
     }
     else
     {
         GameObject.Find("player").GetComponent(playerMovement).enabled = true;
     }
     
     if(!reloading && Input.GetButtonDown("Reload") && currentMag < magSize && currentExtraAmmo > 0)
     {
         reloading = true;
         reloadingAnim.Play(reloadingAnimString);
         reloadingSound.Play();
     }
     if(!reloading && Input.GetButtonDown("Fire1")  && currentMag == 0 && currentExtraAmmo > 0)
     {
         reloading = true;
         reloadingAnim.Play(reloadingAnimString);
         reloadingSound.Play();
     }
     if(reloading && !reloadingAnim.IsPlaying(reloadingAnimString))
     {
         if(currentExtraAmmo >= magSize - currentMag)
         {
             currentExtraAmmo -= magSize - currentMag;
             currentMag = magSize;
         }
         if(currentExtraAmmo < magSize - currentMag)
         {
             currentMag += currentExtraAmmo;
             currentExtraAmmo = 0;
         }
         reloading = false;
     }
     
     if(currentMag > magSize)
         currentMag = magSize;
     if(currentExtraAmmo > maxExtraAmmo)
         currentExtraAmmo = maxExtraAmmo;
     if(currentMag < 0)
         currentMag = 0;
     if(currentExtraAmmo < 0)
         currentExtraAmmo = 0;
     
     var holdSound : GameObject;
     var holdMuzzleFlash : GameObject;
     
     if(Input.GetButtonDown("Fire1") && currentMag > 0 && !reloading)
     {
         if(waitTillNextFire <= 0)
         {
             currentMag -= 1;
             if(bullet)
             {
                 Instantiate(bullet, bulletSpawn.transform.position, bulletSpawn.transform.rotation);
             }
             if(bulletSound)
             {
                 holdSound = Instantiate(bulletSound, bulletSpawn.transform.position, bulletSpawn.transform.rotation);
             }
             if(muzzleFlash)
             {
                 holdMuzzleFlash = Instantiate(muzzleFlash, bulletSpawn.transform.position, bulletSpawn.transform.rotation);
             }
             waitTillNextFire = 1;
         }
     }
     waitTillNextFire -= Time.deltaTime * fireSpeed;
     
     if(holdSound)
     {
         holdSound.transform.parent = transform;
     }
     if(holdMuzzleFlash)
     {
         holdMuzzleFlash.transform.parent = transform;
     }
     
 }
 
 function OnGUI ()
 {
         for (var i : int = 1; i <= currentMag; i++)
         {
             GUI.DrawTexture(Rect(ammoStartX + ((i - 1) * (ammoSize.x + ammoSpacing)), ammoY, ammoSize.x, ammoSize.y), bulletHudTexture);
         }
         GUI.Label(ammoCountRect, currentExtraAmmo.ToString());
 }
 function LookAt()
 {
         var rotation = Quaternion.LookRotation(target.position - transform.position);
         transform.rotation = Quaternion.Slerp(transform.rotation, rotation, Time.deltaTime * damping);
 }

And an another

 #pragma strict
 
 var maxDist : float = 1000000000;
 var decalHitWallHole : GameObject;
 var decalHitWallBlood : GameObject;
 var floatInFrontOfWall : float = 0.00001;
 var damage : int = 20;
 
 function Update () 
 {
     var hit : RaycastHit;
     if(Physics.Raycast(transform.position, transform.forward, hit, maxDist))
     {
         if(decalHitWallHole && hit.transform.tag == "Wall")
         {
             Instantiate(decalHitWallHole, hit.point + (hit.normal * floatInFrontOfWall), Quaternion.LookRotation(hit.normal));
         }
         if(decalHitWallBlood && hit.transform.tag == "Enemy")
         {
             Instantiate(decalHitWallBlood, hit.point + (hit.normal * floatInFrontOfWall), Quaternion.LookRotation(hit.normal));
             hit.transform.SendMessage("Damage",damage,SendMessageOptions.DontRequireReceiver);
         }
     }
     Destroy(gameObject);
 }

Thanks you :)