Null Reference Exception transform position y

Hi, Unity Answers Heroes!
I am getting a Null Reference Exception, and although it might be simple to solve for most, any help would be great!
Here is the code:

	private GameObject leftAsteroid = null;
	private GameObject middleAsteroid = null;
	private GameObject rightAsteroid = null;
	// Update is called once per frame
	void Update () {
		countdown -= Time.deltaTime;
		if (countdown == -1.0f) {
			leftAsteroid = Instantiate (Asteroid, Asteroid1);
			middleAsteroid = Instantiate (Asteroid, Asteroid2);
			rightAsteroid = Instantiate (Asteroid, Asteroid3);
             //System.NullReferenceException error on the next line
		if(leftAsteroid.transform.position.y <= -2.0f){
                      //code here

change and try those lines like this :

leftAsteroid = Instantiate(Asteroid, Asteroid1) as GameObject;

Because of the countdown leftasteroid is not spawned yet so it is null.

 if (leftAsteroid != null && leftAsteroid.transform.position.y <= -2.0f) Debug.Log("OK!");