Check order exists in wc_clear_cart_after_payment()

Fixes #6649
Show notice when nonce isn't set. Fixes #6658

Wrap entire process_checkout method in try catch block to make errors
output correctly.
products settings tab. #6639
Fixed the WooCommerce Status widget links, closes #6685
Added keyup event for refunds gateway button
Improved the Shipping class field in quick edit and bulk edit, closes #6681