Hi,
When an order is declined, and you go back through your saved cart, such as press Continue Shopping, then press Checkout, then in the first screen with the blue checkout bar, you select Checkout again, it just goes back to the BuyNow page with an Object reference not set to an instance of an object error at the top of the page as shown in the image below.
If you then empty your cart, and add the product back, you can checkout again. This will happen on any decline, or incomplete paypal purchase after you go to the PayPal website.
Steve