public function __construct( WP_Customize_Manager $manager, $id, array $args = array() ) {
		if ( empty( $manager->nav_menus ) ) {
			throw new Exception( 'Expected WP_Customize_Manager::$nav_menus to be set.' );
		}

		if ( ! preg_match( self::ID_PATTERN, $id, $matches ) ) {
			throw new Exception( "Illegal widget setting ID: $id" );
		}

		$this->post_id = intval( $matches['id'] );
		add_action( 'wp_update_nav_menu_item', array( $this, 'flush_cached_value' ), 10, 2 );

		parent::__construct( $manager, $id, $args );

		// Ensure that an initially-supplied value is valid.
		if ( isset( $this->value ) ) {
			$this->populate_value();
			foreach ( array_diff( array_keys( $this->default ), array_keys( $this->value ) ) as $missing ) {
				throw new Exception( "Supplied nav_menu_item value missing property: $missing" );
			}
		}

	}