missing reference exception random maze generator

im getting a null reference exception when ever i destroy my gameobject then respawn it. i have a random maze generator which i made from a tutorial and made a few alteration to it the first being that it can increase it size and the second is that i can make alteration in the game manager rather then going directly to the maze prefab which is later instantiated in and destroy when i reset the maze.

using UnityEngine; using System.Collections;

public class GameManager : MonoBehaviour {

 public Maze mazePrefab;

 private Maze mazeInstance;

 public IntVector2 sizeGM;

 public GameObject mazeGO;

 public Maze script;

 private float
 mazeScale;
 public float
 scaleUp = 5f;

 void Start () {
     StartCoroutine(BeginGame ());
 }

 void OnDestroy(){
     StopAllCoroutines ();
 }

 private IEnumerator BeginGame(){
     while(true){
         mazeInstance = Instantiate (mazePrefab) as Maze;


         mazeGO = GameObject.FindGameObjectWithTag ("MazeSetup");
         script = mazeGO.GetComponent ("Maze") as Maze;
         script.size = (sizeGM);

         StartCoroutine (mazeInstance.Generate ());
         while (!Input.GetKeyDown (KeyCode.Space) && !mazeInstance.IsFinished()) {
                 yield return null;
             }

         Debug.Log ("startScale");
         Vector3 mazeScale = mazeGO.transform.localScale;
         mazeScale.x += scaleUp;
         mazeScale.y += scaleUp;
         mazeScale.z += scaleUp;
         mazeGO.transform.localScale = mazeScale;
         Debug.Log ("finishScale");

         Destroy (mazeInstance.gameObject);
     }
 }

}

it also destroy and recreate the maze endlessly but doesnt scale the maze up