Custom Page Builder Widget Groups not working

Hello, I discovered Site Origin recently while doing a research on page builders for an upcoming project. I have to say this is a great plugin!

I’m having trouble getting a custom group which I’ve added, following the instructions from this page:
https://siteorigin.com/docs/page-builder/widget-groups/

Adding group with a name of ‘Testing Widgets’:

if ( ! function_exists( 'testing' ) ) :
function testing( $tabs ) {
  $tabs[] = array(
    'title' => __('Testing Widgets', 'siteorigin-panels'),
    'filter' => array(
      'groups' => array('testing')
    )
  );
  return $tabs;
}
endif;
add_action('siteorigin_panels_widget_dialog_tabs', 'testing');

Now the for next part, I have registered a widget which appears in the list of all widgets, but I could not get it to appear inside any group (I’ve tried ‘wordpress’, ‘panels’, etc). Here’s the code:

function __construct() {
      parent::__construct(
          'foo_widget', // Base ID
          __( 'Testing', 'text_domain' ), // Name
          array(
              'description' => __( 'A Foo Widget', 'text_domain' ),
              'panels_groups' => 'testing',
          )
      );
    }

Am I missing something? Thanks in advance.