Adding items in order gives NullReferenceException (WooCommerce, in C# and using WooCommerce.NET addin.

I am trying to use this code for making an order in Woocommerce, using this add-on :https://github.com/XiaoFaye/WooCommerce.NET. My code is this:  

        var order = new Order() { status = "pending" };
        order.currency = "EUR";
        order.line_items.Add(new OrderLineItem()
        {
            id=44,
            product_id = 44,
            quantity=2
        }
          );
        order.customer_id = 1;
 
       Task ord = wc.Order.Add(order);
       ord.Wait();
But everytime it gives me this exception:
System.NullReferenceException: 'Object reference not set to an instance of an object.'

I have tried everything but no success...