Shoot a bullet based on where the mouse cursor is located at?

ok so i am building like a 2d world sort of like paper mario type of world where the player can move in 3 dimensions but a bit like 2d. I want the camera to follow the main character like a side scroller at the same time I want the user to be able to shoot at the enemies through the camera view. So the camera will be fixed at the user and I want the person to be able to shoot based on where they click. So I made a camera class that follows a character and I made a empty object that follows the camera and put this script on it.

 var smooth:int; // Determines how quickly object moves towards position
 var bullet: GameObject;
 var shootforce : float;
 var fireRate : float = 0.5;
 private var nextFire : float = 0.0;
 var reticle : Texture2D;
 
 private var targetPosition:Vector3;
 
 function Update () {
      if (Input.GetButton ("Fire1") && Time.time > nextFire) {
         nextFire = Time.time + fireRate;
         var instanceBullet = Instantiate(bullet, transform.position, Quaternion.identity);
             instanceBullet.rigidbody.AddForce(transform.forward * shootforce);
        
   // var playerPlane = new Plane(Vector3.up, transform.position);
     var ray = Camera.main.ScreenPointToRay (Input.mousePosition);
     var hitdist = 0.0;
    
     if (instanceBullet.Raycast (ray, hitdist)) {
         var targetPoint = ray.GetPoint(hitdist);
         targetPosition = ray.GetPoint(hitdist);
    //     var targetRotation = Quaternion.LookRotation(targetPoint - transform.position);
     //    transform.rotation = targetRotation;
     }
 }
   
 //transform.position = Vector3.Lerp (transform.position, targetPosition, Time.deltaTime * smooth);
 }

I keep getting nullreferenceexception error. Anyone know how to shoot based on where the mouse cursor is need HELP!! lol im a newb at programming THANKS A BUNCH!