function roles_select( $field ) {
$data = array('');
$roles_obj = new WP_Roles();
$roles_names_array = $roles_obj->get_names();
if (!empty($roles_names_array)) { foreach($roles_names_array as $role_slug => $roles) {
$data[$role_slug] = $role_slug;
} }
$field['choices'] = $data;
return $field;
}
add_filter('acf/load_field/name=roles_select', 'roles_select');