NullReferenceException: Object reference not set to an instance of an object

Hi, I'm working on an iphone app and I have solved the problem of having the rigidbody not being part of UnityGameEngine.GameObject... but now I'm stuck with this... anyone able to help? my code right now is:

var bullet  = Instantiate(BulletPrefab,GameObject.Find("SpawnBullet").transform.position,Quaternion.identity) as GameObject;
bullet.rigidbody.AddForce(transform.forward * shootForce);

Either you didn't assign anything to bulletPrefab, or else it's not finding SpawnBullet.