script error help

i have this script :

 var velocita : float = 100;
 var danno : float = 0;
 var target : Transform;
 
 private var posiY : float;
 
 function Start () {
     posiY = transform.position.y;
     transform.rotation.x = 0;
     yield WaitForSeconds (3);
     Destroy(gameObject);
 }
 
 function Update () {
 
     transform.position.y = posiY;
     transform.position += transform.forward * velocita * Time.deltaTime;
 }
 
 function OnCollisionEnter(col : Collision) {
   
   if(col.gameObject.tag == "enemy"){
     col.gameObject.GetComponent(MyNavMesh).armatura -=  danno;
   }    
     Destroy(gameObject);
     
 
     
     
 }

but i have this error on playing game :

NullReferenceException: Object reference not set to an instance of an object proiettile.OnCollisionEnter (UnityEngine.Collision col) (at Assets/script/proiettile.js:23)

how can i fix it? sorry for horrible english, i'm italian (y)