File manager not working in block's tools directory

Developing with Concrete
3

Thanks @JohntheFish

Have now changed the code following this guide, so added a route:

protected $pkgAutoloaderRegistries = array(

        'src/Controller' => 'MyPackage\Controller',

 );

$router = $this->app->make(RouterInterface::class);
$router->get('editing/form', 'MyPackage\Controller\EditingForm::add_edit');

The form modal shows up correctly with the route but still missing the input fields.

Do I need to reload the JS and CSS? Or something else, thanks.