NullReferenceException in Update()

     public Crystal[] crystals; 
     public Transform Sapphire, Ruby;
 
     private Crystal[,] grid;
 
     // Use this for initialization
     void Start()
     {
         Instantiate(Sapphire, new Vector3(1, 1, 0), Quaternion.identity);
         Instantiate(Sapphire, new Vector3(2, 1, 0), Quaternion.identity);
         Instantiate(Ruby, new Vector3(3, 1, 0), Quaternion.identity);
         Instantiate(Ruby, new Vector3(4, 1, 0), Quaternion.identity);
 
 
         Crystal[,] grid = new Crystal[9, 9];
         crystals = FindObjectsOfType<Crystal>(); 
 
         foreach (Crystal i in crystals) {
             grid[i.xPos, i.yPos] = i;
         }
         
     }
 
     // Update is called once per frame
     void Update()
     {
         print(grid[1, 1]);
 }

Hey guys im getting a NullReferenceException Error when i try to run this code in the console and the problem is the print statement in Update. For some reason if i put the statement in Start it works fine but i get the error in Update. Can someone tell me why? Thanks!