Fix incorrect field access when reloading a stripe payment page

Fix incorrect field access when reloading a stripe payment page

Typical case is if an invoice is open for payment in two different tabs,
it's then paid in one tab and then reloaded in the other, the Strip
integration would crash because it accessed the wrong field. This has
been wrong since day one of the Strip integration, so the comment
stating it won't "normally happen" was entirely correct...