Help with array/lists manipulation in Editor script

Hi,

I'm trying to create an arbitrary 2D grid of cubes in an Editor script, and then select random portions of this grid, just like in this picture:

I've tried using a 2D array, but it won't work unless I define its dimensions while declaring it (and it's not what I need, because I need the user to set them). Here's the code using arrays:

 using UnityEngine;

using UnityEditor; using System.Collections; using System.Collections.Generic;

public class RNDGEN : EditorWindow {

 private int mapX, mapY = 0;
 
 GameObject[,] mapArray;
 
 GameObject floorTile;
 
 GameObject baseGrid = null;

 
 
 [MenuItem ("Window/My Window")]
 static void Init () 
 {
     // Get existing open window or if none, make a new one:
     RNDGEN rndGen = (RNDGEN)EditorWindow.GetWindow (typeof (RNDGEN));
 }
 

 void OnGUI()
 {
     mapX = EditorGUI.IntSlider(new Rect(3, 3, position.width - 6, 15), "Map width:", mapX, 2, 50);
     
     mapY = EditorGUI.IntSlider(new Rect(3, 25, position.width - 6, 15), "Map height:", mapY, 2, 50);
     
     mapArray = new GameObject[mapX, mapY];
     
     floorTile = (GameObject)Resources.Load("Floor_Tile");
     
     
     if (GUI.Button(new Rect(3, 45, position.width - 6, 20), "Create Grid"))
     {    
         baseGrid = new GameObject();
         
         baseGrid.name = "Base_Grid";
         
         for (int i = 0; i < mapX; i++)
         {    
             for (int j = 0; j < mapY; j++)
             {
                 GameObject tile = (GameObject)Instantiate(floorTile, new Vector3(i, 0, j), Quaternion.identity);
                 
                 tile.transform.parent = baseGrid.transform;
                 
                 tile.name = "Tile_" + i.ToString() + "-" + j.ToString();
                 
                 mapArray[i, j] = tile;
                 
             }
             
         }
     
     }
     
     
     if (GUI.Button(new Rect(3, 75, position.width - 6, 20), "Test"))
     {        
         for (int i = 0; i < Random.Range(0, mapArray.GetLength(0)); i++)
         {
             for (int j = 0; j < mapArray.GetLength(1); j++)
             {
                 if (mapArray[i, j] != null)
                 {
                     mapArray[i, j].renderer.material.color = Color.red;
                     
                 }
                 
             }
             
         }
         
     }

 }

}

Then I tried using nested lists, but it's not working like it should. Maybe I'm doing something wrong while iterating through the nested lists. Here's the code using lists:

 using UnityEngine;

using UnityEditor; using System.Collections; using System.Collections.Generic;

public class RNDGEN : EditorWindow {

 private int mapX, mapY = 0;
 
 List<List<GameObject>> mapArrayX = new List<List<GameObject>>();
 
 List<GameObject> mapArrayY = new List<GameObject>();
 
 GameObject floorTile;
 
 GameObject baseGrid = null;

 
 
 [MenuItem ("Window/My Window")]
 static void Init () 
 {
     // Get existing open window or if none, make a new one:
     RNDGEN rndGen = (RNDGEN)EditorWindow.GetWindow (typeof (RNDGEN));
 }
 

 void OnGUI()
 {
     mapX = EditorGUI.IntSlider(new Rect(3, 3, position.width - 6, 15), "Map width:", mapX, 2, 50);
     
     mapY = EditorGUI.IntSlider(new Rect(3, 25, position.width - 6, 15), "Map height:", mapY, 2, 50);
     
     
     floorTile = (GameObject)Resources.Load("Floor_Tile");
     
     
     if (GUI.Button(new Rect(3, 45, position.width - 6, 20), "Create Grid"))
     {    
         baseGrid = new GameObject();
         
         baseGrid.name = "Base_Grid";
         
         for (int i = 0; i < mapX; i++)
         {    
             for (int j = 0; j < mapY; j++)
             {
                 GameObject tile = (GameObject)Instantiate(floorTile, new Vector3(i, 0, j), Quaternion.identity);
                 
                 tile.transform.parent = baseGrid.transform;
                 
                 tile.name = "Tile_" + i.ToString() + "-" + j.ToString();
                 
                 mapArrayY.Add(tile);
                 
                 mapArrayX.Add(mapArrayY);
                 
             }
             
         }
     
     }
     
     
     if (GUI.Button(new Rect(3, 75, position.width - 6, 20), "Test"))
     {        
         for (int i = 0; i < Random.Range(0, mapArrayX.Count); i++)
         {
             for (int j = 0; j < mapArrayY.Count; j++)
             {
                 if (mapArrayX[i][j] != null)
                 {
                     mapArrayX[i][j].renderer.material.color = Color.red;
                     
                 }
                 
             }
             
         }
         
     }

 }

}

Any help?