In SelectBillingAddress method of checkout controller we face with an exception if address.Country is null. The code which cause an exception is:
if (_shippingSettings.ShipToSameAddress && shipToSameAddress && _shoppingCartService.ShoppingCartRequiresShipping(cart) && address.Country.AllowsShipping)
We think that checking for address.Country != null is missing.
We submit an issue for this bug here.