Calling a public function
Your current code doesn't work because of namespaces, but even so, that's not the right way to approach your requirement.

AdvancedHelpManager is a plugin manager, which is a service, so you should get the instance of it like so:

$plugin_manager = \Drupal::service('plugin.manager.advanced_help');
$topics = $plugin_manager-&gt;getTopics();


(as always, if you're in a context where DI is available, inject the service instead of calling the static \Drupal method).

If any of this doesn't make sense it would be a very good idea to read the Services and dependency injection docs, which help to explain the architecture behind Drupal 8.