raycast shooting

hello guys i have a script with a raycast shooting part in it but every time i use it it gives the error: NullReferenceException: Object reference not set to an instance of an object MoveTank.Update () (at Assets/P3J/Demo Scene/MoveTank.js:129)

my script:

 #pragma strict
 
 var leftTrack : MoveTrack;
 var rightTrack : MoveTrack;
 
 var acceleration : float = 5;
 
 var currentVelocity : float = 0;
 var maxSpeed : float = 25;
 
 var rotationSpeed : float = 30;
 
 var spawnPoint : Transform;
 var bulletObject : GameObject;
 var fireEffect : GameObject;
 var maxShots :  int  =  1 ;
 var shots :  int  =  0 ;
 public var ReloadTime:float = 2;
 var MaxAmmo : int = 40 ;
 
 var gunSnd : AudioClip;
 var Effect : Transform;
 var TheDammage = 100;
 
 function Start() {
 
     // Get Track Controls
     leftTrack = GameObject.Find(gameObject.name + "/Lefttrack").GetComponent(MoveTrack);
     rightTrack = GameObject.Find(gameObject.name + "/Righttrack").GetComponent(MoveTrack);
     
 }
 
 
 function Update () {
     
     if (Input.GetKey (KeyCode.W)) {
         // plus speed
         if (currentVelocity <= maxSpeed) 
             currentVelocity += acceleration * Time.deltaTime;
 
     } else if (Input.GetKey (KeyCode.S)) {
         // minus speed
         if (currentVelocity >= -maxSpeed) 
             currentVelocity -= acceleration * Time.deltaTime;
         
     } else {
         // No key input. 
         if (currentVelocity > 0) 
             currentVelocity -= acceleration * Time.deltaTime;
         else if (currentVelocity < 0) 
             currentVelocity += acceleration * Time.deltaTime;
 
     }
 
 
     // Turn off engine if currentVelocity is too small. 
     if (Mathf.Abs(currentVelocity) <= 0.05)
         currentVelocity = 0;
 
     // Move Tank by currentVelocity
     transform.Translate(Vector3(0, 0, currentVelocity * Time.deltaTime));
 
     // Move Tracks by currentVelocity     
     if (currentVelocity > 0) {
         // Move forward
         leftTrack.speed = currentVelocity;
         leftTrack.GearStatus = 1;
         rightTrack.speed = currentVelocity;
         rightTrack.GearStatus = 1;
     }
     else if (currentVelocity < 0)    {
         // Move Backward
         leftTrack.speed = -currentVelocity;
         leftTrack.GearStatus = 2;
         rightTrack.speed = -currentVelocity;
         rightTrack.GearStatus = 2;
     }
     else {
         // No Move
         leftTrack.GearStatus = 0;    
         rightTrack.GearStatus = 0;        
     }
 
 
     // Turn Tank
     if (Input.GetKey (KeyCode.A)) {
         if (Input.GetKey(KeyCode.DownArrow)) {
             // Turn right
             transform.Rotate(Vector3(0, rotationSpeed * Time.deltaTime, 0));
             
             leftTrack.speed = rotationSpeed;
             leftTrack.GearStatus = 1;
             rightTrack.speed = rotationSpeed;
             rightTrack.GearStatus = 2;
             
         } else {
             // Turn left
             transform.Rotate(Vector3(0, -rotationSpeed * Time.deltaTime, 0));
             
             leftTrack.speed = rotationSpeed;
             leftTrack.GearStatus = 2;
             rightTrack.speed = rotationSpeed;
             rightTrack.GearStatus = 1;
             
         }
     }
 
     if (Input.GetKey (KeyCode.D)) {
         if (Input.GetKey(KeyCode.DownArrow)) {
             // Turn left
             transform.Rotate(Vector3(0, -rotationSpeed * Time.deltaTime, 0));
             leftTrack.speed = rotationSpeed;
             leftTrack.GearStatus = 2;
             rightTrack.speed = rotationSpeed;
             rightTrack.GearStatus = 1;
 
         } else {
             // Turn right
             transform.Rotate(Vector3(0, rotationSpeed * Time.deltaTime, 0));
             leftTrack.speed = rotationSpeed;
             leftTrack.GearStatus = 1;
             rightTrack.speed = rotationSpeed;
             rightTrack.GearStatus = 2;
             
         }
     }
     
     var hit : RaycastHit;
     var ray : Ray = Camera.main.ScreenPointToRay(Vector3(Screen.width*0.5, Screen.height*0.5, 0));    
     
     // Fire!
     if (Input.GetButtonDown("Fire1") && shots < maxShots && MaxAmmo > 0) {
         if (Physics.Raycast (ray, hit, 100))
         {
             var particleClone = Instantiate(Effect, hit.point, Quaternion.LookRotation(hit.normal));
             Destroy(particleClone.gameObject, 2);
             hit.transform.SendMessage("ApplyDammage", TheDammage, SendMessageOptions.DontRequireReceiver);
         }
         shots ++;
          Reload();
          MaxAmmo --;
     }
 }
 
 function Reload(){
  yield WaitForSeconds(ReloadTime);///Wait 2(ReloadTime) seconds before continuing
  shots = 0;
  
  }
  
  function OnGUI () {
     GUI.Label (Rect (750, 25, 100, 30), MaxAmmo.ToString() + " Shells left");
     GUI.Label (Rect (750, 50, 150, 30), ReloadTime.ToString() + " Sec reload time");
 }