RaycastHit.rigidBody.gameObject is nullReferenceExeption

I try to Hit GameObject with this code

var ray = Camera.main.ScreenPointToRay(new Vector3(Camera.main.pixelWidth / 2,    Camera.main.pixelHeight/2));

        RaycastHit hit;
        if (Physics.Raycast(ray,out hit,100))
        {
            print(hit.rigidbody.gameObject); //And make NullReferenceExeption error in this point
        }

All game object i create with this code

   public Object GrassPrefab;

    // Use this for initialization
    void Start ()
    {
        //    
        for (int x=0;x<20;x++)
            for (int z = 0; z < 20; z++)
            {
                Instantiate(GrassPrefab,new Vector3(x,0,z),Quaternion.identity);
            }
    }

Make sure your grass prefab object, whatever it is, has a collider for the ray to hit.