Source

File: wp-includes/customize/class-wp-customize-nav-menu-setting.php

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->term_id = intval( $matches['id'] );

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