Instantiating from an object classes GameObject property?

Right now I have a base class called Tile that doesn't derive from monobehaviour. It has a private GameObject with public getter and setter. I have another class called StoneTile that inherits from Tile where I set the GameObject via Resouces.Load();. Then I have an actual game script which creates an instance of StoneTile and instantiates StoneTile's GameObject... But it causes a NullRefrenceException. I'm trying this new method of clean code and inheritance so I'm a bit lost. Any tips or ideas will help. Thanks

 using UnityEngine;
 using System.Collections;
 
 public class Tile
 {
     private string type = "TILE";
     private float speedBoost = 0f;
     private float jumpBoost = 0f;
     private float clickIncrease = 0.1f;
     private float naturalIncrease = 0.1f;
     private GameObject go;
 
 
     public string Type
     {
         get
         {
             return type;
         }
         set
         {
             type = value;
         }
     }
     
     public float SpeedBoost
     {
         get
         {
             return speedBoost;
         }
         set
         {
             speedBoost = value;
         }
     } 
 
     public float JumpBoost
     {
         get
         {
             return jumpBoost;
         }
         set
         {
             jumpBoost = value;
         }
     }
 
     public float ClickIncrease
     {
         get
         {
             return clickIncrease;
         }
         set
         {
             clickIncrease = value;
         }
     }
 
     public float NaturalIncrease
     {
         get
         {
             return naturalIncrease;
         }
         set
         {
             naturalIncrease = value;
         }
     }
 
     public GameObject GO
     {
         get
         {
             return go;
         }
         set
         {
             go = value;
         }
     }
 }

~~~~~

 using UnityEngine;
 using System.Collections;
 
 public class StoneTile : Tile
 {
     public StoneTile()
     {
         Type = "Stone";
         SpeedBoost = 0.2f;
         JumpBoost = 0;
         ClickIncrease = 0.2f;
         NaturalIncrease = 0.1f;
         GO = Resources.Load("Stone") as GameObject;
     }
 }

~~~~

 using UnityEngine;
 using System.Collections;
 
 public class WorldCreate : MonoBehaviour {
 
     
     public int width;
     public int height;
     public Tile[,] map;
      
 
     void Start()
     {
         map = new Tile[width, height];
         GenerateMap();
         Initialize();
     }
 
 
     void GenerateMap()
     {
         for (int x = 0; x < width; x++)
         {
             for (int y = 0; y < height; y++)
             {
                 float r = Random.value;
                 if (r <= 0.25f)
                 {
                     map[x, y] = new StoneTile();
                 }
             }
         }
     }
 
     void Initialize()
     {
         for (int x = 0; x < width; x++)
         {
             for (int y = 0; y < height; y++)
             {
                 GameObject newTile = Instantiate(map[x,y].GO);    ***<---- Error Here***
                 newTile.transform.position = new Vector3(x, 0, y);
             }
         }
     }
 }