Unassigned Reference Exception Error in Space Shooter

I have followed this tutorial to the letter and its now telling me that the restart text isn’t assigned. but, it is. I’m very confused.
This is my GameController code.

using UnityEngine;
using System.Collections;

public class GameController : MonoBehaviour 
{
	public GameObject hazard;
	public Vector3 spawnValues;
	public int hazardCount;
	public float spawnWait;
	public float startWait;
	public float waveWait;

	public GUIText scoreText;
	public GUIText restartText;
	public GUIText gameOverText;

	private bool gameOver;
	private bool restart;
	private int score;
	
	void Start ()
	{
		gameOver = false;
		restart = false;
		restartText.text = "";
		gameOverText.text = "";
		score = 0;
		UpdateScore ();
		StartCoroutine (SpawnWaves ());
	}

	void Update ()
	{
		if (restart)
		{
			if (Input.GetKeyDown (KeyCode.R)) 
			{
				Application.LoadLevel (Application.loadedLevel);
			}
		}
	
	}

	IEnumerator SpawnWaves () 
	{	
		yield return new WaitForSeconds (startWait);
		while (true) 
		{
			for (int i = 0; i < hazardCount; i++) 
				{
					Vector3 spawnPosition = new Vector3 (Random.Range (-spawnValues.x, spawnValues.x), spawnValues.y, spawnValues.z);
					Quaternion spawnRotation = Quaternion.identity;
					Instantiate (hazard, spawnPosition, spawnRotation);
					yield return new WaitForSeconds (spawnWait);
				}
			yield return new WaitForSeconds (waveWait);

			if (gameOver)
			{
				restartText.text = "Press 'R' for Restart";
				restart = true;
				break;
			}
		}
	}

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

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

	public void GameOver ()
	{
		gameOverText.text = "Game Over!";
		gameOver = true;
	}
}

Anyone have any idea?

UnassignedReferenceException: The variable restartText of GameController has not been assigned.
You probably need to assign the restartText variable of the GameController script in the inspector.
GameController.Start () (at Assets/Scripts/GameController.cs:25)

This is the whole error message.
And yes i dragged it over. And its still there.