How can I push user e-mail to checkout?

[Sterling-Malory-Archer]

As stated in the title, I want to push the user's email into the checkout form when I open using .GetWebCheckoutLink.

I tried to use cartState.SetEmailAddress, but I am getting a null reference exception being thrown.

 public void AddItemToCart()
    {
        //Reset the cart because there's a bug with it not updating if the users open it when it's empty.
        cart.Reset();

        // Try to set the user e-mail in the checkout.
        cartState.SetEmailAddress(tMP_InputField.text,
            (error) =>
            {
            });

        //DefaultQueries.checkout.EmailUpdate(query, CurrentCheckout.id(), email);

        //For every cartItem in the List of CartItems, we add it to the cart after we reset it.
        foreach (var cartItem in cartItems)
        {
            cart.LineItems.AddOrUpdate(cartItem, 1);
        }
    }

Any help is kindly appreciated!

EDIT:
This is the error I am getting:

And this is the line it's referring to (Where my horizontal cursor is):