Hi all, i try to do my first shoot but i have one problem (I am new in Unity3D).

i Have this code:

public var puntoDisparo: Transform;
public var bala: GameObject;
public var velocidad: float = 10;

function Update () {

    if(Input.GetButtonDown("Fire1")){

       var cloneBala : GameObject = Instantiate (bala, puntoDisparo.position, Camera.main.transform.rotation);
       cloneBala.rigidbody.AddForce(new Vector3(0,0,velocidad), ForceMode.velocityChange);
    }

}

Before add this line cloneBala.rigidbody.AddForce(new Vector3(0,0,velocidad), ForceMode.velocityChange); When i clic on left button mouse my gameObject is perfectly duplicated but however when i try to do that this gameObject can move foward i have this error:

NullReferenceException: Object reference not set to an instance of an object Disparo.Update () (at Assets/Scripts/Disparo.js:15)

'Disparo' is the script that i show before.. So.. What happend? my gameObject only move when i push it with others gameObjects (same gameObjects)

Here some pics.. In the Screen Shot UnityShoot you can see that only when i clicked 3 times if when one gameobject fly! :(

Please Help i want to keep learning

ScreenShots 125125

http://www.dropbox.com/gallery/45494271/1/Screen%20Shots%20Unity3D%20FPS?h=317b52