Hi and thank you for your report.
This has been fixed in FPX add-on version 1.0.4, please update it.
I have notified the team that FPX add-on should be specifically tested against any changes in core plugin in order to prevent similar situations from happening in the future. The issue with testing it is that FPX requires Stripe account registered in Malaysia (even for testing purposes!), which is an issue when you’re not from Malaysia. But this is sorted now, we have been provided with test keys by Malaysian Stripe team.
I apologies for inconveniences this may have caused.