[PHP 7.3] "session_id(): Cannot change session id"

After moving from PHP 5.6 to PHP 7.2, this warning shows up in my logs (admin/reports/dblog):

Warning: session_id(): Cannot change session id when headers already sent in drupal_session_initialize()
(Line 266 of ... /includes/session.inc).

I couldn't find anything on this other than an issue in Durpal 8. Did anyone else run into this?

Line 266 of session.inc:

    session_id(drupal_random_key());
    if ($is_https && variable_get('https', FALSE)) {
      $insecure_session_name = substr(session_name(), 1);
      $session_id = drupal_random_key();
      $_COOKIE[$insecure_session_name] = $session_id;
    }