Disabling a GameObject and Enabling another when int reaches a certian number.

Hello, I am currently having some major issues with this script. Essentially, I am trying to increase the games difficulty over time by disabling and enabling certain game objects. For example, disable and enable one at 5/10/20/50.

public class ScoreController : MonoBehaviour {

	private float score;
	public GUIText scoreText;

	private GameObject asteroidController5;
	private GameObject asteroidController15;

	void Start()
	{
		score = 0;

		asteroidController5 = GameObject.FindWithTag("ASC0-5");	
		asteroidController15 = GameObject.FindWithTag("ASC5-15");

	}

	public void AddScore(int newScoreValue)
	{
		score += newScoreValue;
		UpdateScore();
	}

	void UpdateScore()
	{
		scoreText.text = "Score: " + score;
	}

	void Update()
	{
		if(score == 5)
		{
			asteroidController5.SetActive(false);
			asteroidController15.SetActive(true);

		}


	}
}

So when the player reaches a score of 5, I am trying to disable asteroidController5 and replace it with asteroidController15 but I am getting this error message:

object reference not set to an instance of an object

Could anyone help me? Thank you! (I can disable asteroidController5 but cannot enable asteroidController15, asteroidController15 is disabled in the inspector in this helps.)

Debug to make sure you have all objects found and assigned

void Start()
{
score = 0;

    asteroidController5 = GameObject.FindWithTag("ASC0-5");
    if( asteroidController5 == null ) Debug.Log("Game object asteroidController5 not found );
    asteroidController15 = GameObject.FindWithTag("ASC5-15");
    if( asteroidController15 == null ) Debug.Log("Game object asteroidController15 not found );
    else asteroidController15.SetActive( false );

}


 void Update()
{
    if(score == 5)
    {
        if( asteroidController5 != null )asteroidController5.SetActive(false);
else Debug.Log("Game object asteroidController5 not assigned );
        if( asteroidController15 != null )asteroidController15.SetActive(true);
else Debug.Log("Game object asteroidController15 not assigned);
 
    } 
}

The GameObject.Find methods will not find deactivated objects. You need to either have them activated while you search for them, and deactivate them in the script, or make a public field that can be assigned to the class instance in the inspector.