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"

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

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"

EDIT

'walker' => new $walker, should probably be something like 'walker' => $walker ? new $walker : '', to avoid errors, if $walker is null.

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

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

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"

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