**Update 30.6.2019**

Ah, now I think understand what is going on with the code and the warning. I think it is literally trying to find a class named "new wp_bootstrap_navwalker()".

In my local sandbox WP I got the shortcode working after I changed `'walker' => $walker,` to `'walker' => new $walker,`. I also changed the walker parameter from `walker="new wp_bootstrap_navwalker()"` to `walker="wp_bootstrap_navwalker"`

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If you're getting a "class not found" warning, then maybe the class file hasn't just been included/loaded when you're trying to instantiate the class. So perhaps you could try requiring/including the class file inside your shortcode function when it's needed. Something along these lines,

    function se_menu_shortcode( $atts, $content = null ) {
    
      // extract
    
      if ( ! class_exists('wp_bootstrap_navwalker') ) {
        require_once 'path/to/class/definition/file.php';
      }
    
      // return
    }

Or maybe you could try writing an autoloader for handling the requiring/including of classes automatically, https://www.php.net/manual/en/function.spl-autoload-register.php