<?php

// Error: Notice: Trying to get property 'post_name' of non-object in /path/to/wordpress/functions.php on line 616

function change_post_types_slug( $args, $post_type ) {


  if ( 'lead_page' === $post_type ) {

    $id = isset($_GET[ 'post' ]) ? $_GET[ 'post' ] : '';
    $location = get_field('leadpage_location', $id);
    $args['rewrite']['slug'] = $location->post_name;

  }

  return $args;
}

add_filter( 'register_post_type_args', 'change_post_types_slug', 10, 2 );