What is the correct way to call a function in a class?

Question

C++ / C#

Hello. I have an AddItem method in the Inventory class

public void AddItem(int id, Item item, int count)
    {
        items[id].id = item.id;
        items[id].count = count;
        items[id].itemGameObject.GetComponent<Image>().sprite = item.img;
        if (count > 1 && item.id != 0)
        {
            items[id].itemGameObject.GetComponentInChildren<Text>().text = count.ToString();
        } else
        {
            items[id].itemGameObject.GetComponentInChildren<Text>().text = "";
        }
    }

I am in another file, in the RockTrigger class, trying to add an item to this inventory system of mine

Inventory inv = new Inventory();
inv.AddItem(0, inv.items[1], 1);

In Unity itself, I set items for the items variable

and when using inv.AddItem(0, inv.items[1], 1); I get the following error:
ArgumentOutOfRangeException: Index was out of range. Must be non-negative and less than the size of the collection.
Parameter name: index
As I understand it, it considers that there is no item with id 1 in items. But it's not. What am I doing wrong?

Update:
I made a mistake, I should have used inv.data.items[1]. data is taken from another file and class

public class DataBase : MonoBehaviour
{
    public List<Item> items = new List<Item>();
}

And the error is accordingly different
NullReferenceException: Object reference not set to an instance of an object
RockTrigger.Update () (at Assets/Scripts/RockTrigger.cs:26)

The same line 26:
inv.AddItem(0, inv.data.items [eleven);

Answer 1

inv.AddItem(0, inv.items[1], 1);
You've just created an empty inventory, but you're trying to add (though code-replace) the null element as the first element.
Where does the first element to replace come from?
Where does the null element come from?