Resource error: Object reference not set to an instance of an object

Question

I am trying to convert my inventory system from unity to Godot and when I try and grab an item in the inventory it gives me this error, I'm unsure why this is or how to fix it. I'm aware what the error means but I don't understand why its giving me it as it's a reference script not an Object? Any help would be appreciated

Inventory.cs

[Serializable]
public class ItemSlot : Resource
{
public ItemResource item;
public int count;

public void Copy(ItemSlot slot)
{
    this.item = slot.item;
    this.count = slot.count;
}

public void Set(ItemResource item, int count)
{
    this.item = item;
    this.count = count;
}

public void Clear()
{
    this.item = null;
    this.count = 0;
}
}
public class Inventory : Resource
{
   [Export] public List<ItemSlot> inventoryItems = new List<ItemSlot>();

GameManager.cs

public class GameManager : Node
{
    public static GameManager instance;
    internal Inventory inventory;

    public override void _Ready()
    {
        inventory = ResourceLoader.Load<Inventory>("res://Player/Inventory.tres");
    }

Toolbar.cs

 public class Toolbar : Node2D
 {
        public ItemResource GetItem
        {
            get
            {
                //Error occurs here:
                return GameManager.instance.inventory.inventoryItems[selectedTool].item;
            }
         }
  }

Error Output:
E 0:00:00.790 ItemResource Toolbar.getGetItem(): System.NullReferenceException: Object reference not set to an instance of an object.
<C++ Error> Unhandled exception
<C++ Source> C:\Users\user\Documents\Godot\Projects\Assets\Scripts\Player\Toolbar.cs:28 @ ItemResource Toolbar.getGetItem()()
Toolbar.cs:28 @ ItemResource Toolbar.getGetItem()()
Toolbar.cs:80 @ void Toolbar.Equip()()
Toolbar.cs:19 @ void Toolbar.Ready()()