Webform with existing contact results in undefine index
It looks like a fix for this issue has been committed to the webform_civicrm project and should be present in version 7.x-4.21+

Details are here: https://www.drupal.org/project/webform_civicrm/issues/2966760

The fix itself is here: https://github.com/colemanw/webform_civicrm/pull/139