NullReferenceException: Object reference not set to an instance of an object

Hello Unity Community i have an error everytime i destroy the enemy object within the game:

NullReferenceException: Object reference not set to an instance of an object Gui track taret.Dead () (at Assets/Scripts/Gui track taret.js:17) Gui track taret.Update () (at Assets/Scripts/Gui track taret.js:40)

i'v tried almost everything but it doesn't seem to like calling a function in my quest script, Anyways heres the 2 script:

Gui track taret:

     var Quest1 : Quest1;
     var other : KillSys;
     var HealthBar : Transform;
     private var MaxHealth : float = 100;
     private var MaxBar : float = 0.002;
     var Health = 100;
     var Enemy : GameObject;
     //var DieAnimation : AnimationClip;
       
       function Dead()
     {
     //animation.Play("DieAnimation");
         other.KillCounter();    
         Destroy (Enemy);
         Quest1.Kills();
     }   
     
     function Update()
     {
     // --- Change Size Of Bar --- \\
     transform.localScale.z = (MaxBar) * (Health/MaxHealth);
      
     // --- Change Colour Of Bar --- \\
     if(transform.localScale.z == 0.0001)
     {
     transform.renderer.material.color = Color.green;
     }
     if(transform.localScale.z <= 0.0003)
     {
     transform.renderer.material.color = Color.yellow;
     }
     if(transform.localScale.z <= 0.0005)
     {
     transform.renderer.material.color = Color.red;
     }
     if(Health <= 0)
     {
     Dead();
     }
 }
 function ApplyDammage (TheDammage : int)
 {
     Health -= TheDammage;
 }

and heres the Quest1 script:

 var Killstxt = "Mines Killed: ";
 var MaxKills = 5;
 static var CurKills = 0;
 
 function OnGUI () {
 GUI.Box (Rect (Screen.width*0.9-51, 200, 150, 50), Killstxt + CurKills + " / " + MaxKills);
 }
 
 public function Kills(){
 CurKills += 1;
 }