Getting NullReferenceException but object is not null

Question

I'm trying to save and Load on my unity game on a file, I have no problem storing things like highScore, gold etc, but I can't store a list of a class called ObjectShop, I'm getting nullreference error.

I have a code where I can buyItems, they are objects with class ObjectShop, there I have all the information I need.

So when the buton is pressed I'm trying to add to the list the updated object for later store it in to my saveSystem, but getting an error on this line:

SystemDataController.systemDataController.powers.Add(itemListed);

Here is full script:

using System.Collections;
using System.Collections.Generic;
using UnityEngine;
using UnityEngine.UI;

public class BuyButton : MonoBehaviour
{
    public int itemId;

    public void BuyItem()
    {
        foreach (var itemListed in ArmorShop.armorShop.items)
        {
            //Can buy the item
            if (itemListed.id == itemId && !itemListed.bought && GameControllerShop.gameControllerShop.RequestGold() >= itemListed.price)
            {
                //Set it to bought
                itemListed.bought = true;
                //Quit the money
                GameControllerShop.gameControllerShop.RemoveGold(itemListed.price);

                //Here I have the error: NullReferenceException: Object reference not set to an instance of an object  
                SystemDataController.systemDataController.powers.Add(itemListed);
                SystemDataController.systemDataController.SaveData();
            }
        }

        ArmorShop.armorShop.UpdateItem(itemId);
    }
}

and that's what I have so far:

GameData.cs:

using System.Collections;
using System.Collections.Generic;
using UnityEngine;

[System.Serializable]
public class GameData
{
    public float gold;
    public float highScore;
    public List<ObjectShop> powers = new List<ObjectShop>();

    public GameData(SystemDataController game)
    {
        highScore = game.highScore;
        gold = game.gold;
        powers = game.powers;
    }
}

SystemDataController.cs:

using System.Collections;
using System.Collections.Generic;
using UnityEngine;

public class SystemDataController : MonoBehaviour
{
    public static SystemDataController systemDataController;
    public float gold;
    public float highScore;
    public List<ObjectShop> powers = new List<ObjectShop>();

    private void Awake()
    {
        systemDataController = this;
        LoadData();
    }

    public void SaveData()
    {
        SaveSystem.SaveData(this);
    }

    public void LoadData()
    {
        GameData data = SaveSystem.LoadGame();

        if (data != null)
        {
            gold = data.gold;
            highScore = data.highScore;
            powers = data.powers;
        }
    }
}

EDIT: I have read the link provided from here What is a NullReferenceException, and how do I fix it? and it doesn't talk about my problem, mine its about saving a custom class list on a file and also adding items to a possible null list?

EDIT2: Added object shop:

using System.Collections;
using System.Collections.Generic;
using UnityEngine;

[System.Serializable]
public class ObjectShop
{
    public int id;
    public float price;
    public string name;
    public Sprite itemSprite;
    public bool bought;
    public string description;
    public bool equipped;
}