PHP notice - Undefined index: cart_order in uc_cart_checkout_form_cancel()

To replicate: Go to checkout from the cart and select cancel right away.

The function uc_cart_checkout_form_cancel() tries to use the value of $_SESSION['cart_order'] to determine if an order number has been created yet. $_SESSION['cart_order'] will only have been set if uc_cart_checkout_form_validate() ran. Cancelling an order doesn't need / call uc_cart_checkout_form_validate() so unless the 'Review order' button was clicked before cancelling, the session value will not exist.

I left intval($_SESSION['cart_order']) > 0 don't think it's needed though?