Struggling with moving objects using array

Hey guys, I'm really struggling to get this to work. It's a match three style block puzzle game and at the moment I just want to move my blocks down to the bottom of a grid.

 using System.Collections;
 using System.Collections.Generic;
 using UnityEngine;
 
 public class BoardMovement : MonoBehaviour
 {
 
     public Transform[,] grid = new Transform[columns, rows];
     public static int rows = 9;
     public static int columns = 8;
 
 
 
     // Start is called before the first frame update
     void Start()
     {
 
     }
 
     // Update is called once per frame
     void Update()
     {
         if (FindObjectOfType<BlockMovement>().droppedBlock == true)
         {
             updateGrid();
         }
 
         void updateGrid()
         {
 
                 for (int x = 0; x < rows; ++x)
                 {
                     for (int y = 0; y < columns; ++y)
                     {
                         if (grid[x, y] != null)
                         {
                             
                             grid[x, y - 1] = grid[x, y];
                             grid[x, y] = null;
                             grid[x, y - 1].position += new Vector3(0, -1, 0);

                         }
                     }
                 }
             
         }
     }
 }

So droppedBlock returns true when a group of blocks have been placed in the top row. I just want to move those blocks down to the grid floor. Currently it's throwing 'IndexOutOfRangeException' errors and not moving anything.