Undefined index: reject_button_enabled, reject_button_label, close_button_enabled and close_button_action

Thanks for the patch @lily.yan. Can we please not introduce incompatible changes with PHP 5.6 and below intentionally? The "null coalescing operator" was added in PHP 7.0 and therefore it will not work with PHP 5.6 and below, which Drupal 7 still supports. I think it would be better to add a classic isset check.

Created an MR with proposed changes from #3.

Thanks! Code is looking good.

Merged. Thanks for contributing!

Automatically closed - issue fixed for 2 weeks with no activity.