Pre WP 4.9

if( ! function_exists( \'fix_no_editor_on_posts_page\' ) ) {

    /**
    * Add the wp-editor back into WordPress after it was removed in 4.2.2.
    *
    * @param Object $post
    * @return void
    */
    function fix_no_editor_on_posts_page( $post ) {
        if( isset( $post ) && $post->ID != get_option(\'page_for_posts\') ) {
            return;
        }

        remove_action( \'edit_form_after_title\', \'_wp_posts_page_notice\' );
        add_post_type_support( \'page\', \'editor\' );
    }
    add_action( \'edit_form_after_title\', \'fix_no_editor_on_posts_page\', 0 );
}

WP 4.9

if( ! function_exists( \'fix_no_editor_on_posts_page\' ) ) {

    function fix_no_editor_on_posts_page( $post_type, $post ) {
        if( isset( $post ) && $post->ID != get_option(\'page_for_posts\') ) {
            return;
        }

        remove_action( \'edit_form_after_title\', \'_wp_posts_page_notice\' );
        add_post_type_support( \'page\', \'editor\' );
    }

    add_action( \'add_meta_boxes\', \'fix_no_editor_on_posts_page\', 0, 2 );

}

Full Details Here