NullReferenceException - Object not set to an instance

Hi All,

I'm having some trouble generating a new instance of an object. The code worked fine on Start but when called up in an Update function seems to go awry. I'm only new to c# and Unity so am using this project to learn, would appreciate anyone's insight into why this might be happening.

 using UnityEngine;
 using System.Collections;
 using System.Collections.Generic;
 public class Mask : MonoBehaviour 
 {
     public GameObject sphere;
     public GameObject selector;
     public bool isSelected = false;
     string[] maskMats = {"BLUE","GREEN","ORANGE","PURPLE","RED","YELLOW"};
     public string color="";
     public List<Mask> Neighbors = new List<Mask> ();
     public bool isMatched = false;
 
     void Start () 
     {
         CreateMask ();
     }
     
 
     public void CreateMask()
     {
         color = maskMats[Random.Range(0, maskMats.Length)];
         SpriteRenderer spriteRenderer = sphere.AddComponent<SpriteRenderer> ();
         spriteRenderer.sprite = Resources.Load<UnityEngine.Sprite> ("Materials/" + color);
         isMatched = false;
     }
 
 

The update function below is in another script:

 void Update () 
         {
             if (isMatched) 
             {
                 for(int i=0;i<masks.Count;i++)
                 {
                     if(masks[i].isMatched)
                     {
                         masks[i].CreateMask();
                         masks[i].transform.position=new Vector3(
                             masks[i].transform.position.x,
                             masks[i].transform.position.y + 6,
                             masks[i].transform.position.z);
                     }
                 }
     
                 isMatched = false;
             }