CS:GO Trading bot error message: Value cannot be null at System.Linq.Enumerable.ToList[TSource]

Question

I'm working on a steambot for cs:go items. It is a modded version of JesseCar96's steambot for TF2. The Items are stored in a class called schema. Within that there is an item array. Right now I am using this code to get the items in the trade:

    Schema itemSchema = new Schema();
    List<Schema.Item> items;
    items = itemSchema.GetItems();

But when I call this function I get this error message: http://gyazo.com/b543a78ba531e659162a0564e16ba572

My GetItems() function is:

public List<Item> GetItems()
    {
        return Items.ToList();
    }

And my items class is defined as:

[JsonProperty("items")]
public Item[] Items { get; set; }

Any help is greatly appreciated!