Saving admin invite fails

[sergeychernyshev]

Without subscription plans set, sending admin invite fails complaining about

PHP Notice:  Undefined index: plan_slug in .../users/admin/invitations.php on line 34, referer: .../users/admin/invitations.php
PHP Catchable fatal error:  Argument 1 passed to Invitation::setPlan() must be an instance of Plan, boolean given, called in .../users/admin/invitations.php on line 34 and defined in .../users/classes/Invitation.php on line 578, referer: .../users/admin/invitations.php