In PHP 7.4 they have changed the way to report this, if you use PHP 7.3 this notice is not shown.
Use PHP's @ control operator to hide it.
Change line 1131 as follows:
isset($in['notify']) ? $in['notify'] : qa_opt('notify_users_default'), $in['email'], @$errors['email'], $prefix);
to
isset($in['notify']) ? $in['notify'] : qa_opt('notify_users_default'), @$in['email'], @$errors['email'], $prefix);