this Bullet shooting script error

Hey guys , i am trying to make a gun shoot but a error keeps coming up NullReferenceException: Object reference not set to an instance of an object Boo.Lang.Runtime.RuntimeServices.InvokeBinaryOperator (System.String operatorName, System.Object lhs, System.Object rhs) bullet.FixedUpdate () (at Assets/Scripts/bullet.js:15) Here's the code

    // The speed the bullet moves 
    var Speed : float = .4; 
    
    // The number of seconds before the bullet is automatically destroyed 
    var SecondsUntilDestroy : float = 10; 
    
    private var startTime : float; 
    
    function Start () {
        startTime = Time.time; 
    } 
    
    function FixedUpdate () {
        // Move forward 
        this.gameObject.transform.position += Speed * this.gameObject.transform.left;
        
        // If the Bullet has existed as long as SecondsUntilDestroy, destroy it 
        if (Time.time - startTime >= SecondsUntilDestroy) {
            Destroy(this.gameObject);
        } 
    }
         
    function OnCollisionEnter(collision : Collision) {
        
        // Remove the Bullet from the world 
        Destroy(this.gameObject); 
    }

Well guys please respond asap. I dont know what's wrong.