PHP notices should not prevent the final step from displaying on the new template

[lindseymacmillan]

User Story

As a user, I want to the new Multi-Step form to process stripe donations, even when WP DEBUG is enabled, so that I can complete my donation as expected.

Details

After completing a Stripe payment in a new form template, the loader animation just spins without completing transactions.

• @rickalday in associated Canny post

Expected Behavior

Steps to Reproduce

1. Create a new form using the new form template.
2. Set up Stripe
3. Important: In your wp-config.php file enable debug and set WP_DEBUG_DISPLAY to true.
4. Make a test donation.
5. The loader iframe will display indefinitely.

Visuals

Video explanation: http://somup.com/cYjroh67kG

Additional Context

This is because the content iframe returns a bunch of PHP notices but it's hidden from view.
PHP Deprecated: array_key_exists(): Using array_key_exists() on objects is deprecated. Use isset() or property_exists() instead in /Users/daro/Local Sites/newgive/app/public/wp-content/plugins/give/vendor/stripe/stripe-php/lib/ApiRequestor.php on line 402
PHP Warning: Cannot modify header information - headers already sent by (output started at /Users/daro/Local Sites/newgive/app/public/wp-content/plugins/give/vendor/stripe/stripe-php/lib/ApiRequestor.php:402) in /Users/daro/Local Sites/newgive/app/public/wp-includes/pluggable.php on line 1299
Possibly related to the Stripe PHP library: stripe/stripe-php#814

https://givewp.canny.io/admin/board/bug-reports/p/php-notices-should-not-prevent-the-final-step-from-displaying-on-the-new-templat

Acceptance Criteria

• With WP DEBUG enabled, donor is able to complete donations
• No PHP notices appear in the Multi-Step form when processing Stripe donations