Unable to get a simple plugin admin page to work

Well, as it usually goes, after banging my head on this for hours yesterday and then finally deciding to ask for help, I was able to solve it within a few minutes. Here is what I needed to change:

    public function init() {
        // add_action('admin_menu', array($this, 'add_plugin_admin_menu'));
        $this-> add_plugin_admin_menu();
    }

Clearly, I have much to learn about the call chain in WP.