PHP Warning on Login page.

[andrewlimaza]

Describe the bug
Notice: Undefined index: rp_login in /srv/htdocs/wp-content/plugins/paid-memberships-pro/includes/login.php on line 619 (https://github.com/strangerstudios/paid-memberships-pro/blob/dev/includes/login.php#L619)

Reported via user on WordPress.org - https://wordpress.org/support/topic/php-errors-originating-from-pmpro/