cx-newsletter/views/3-manage_messages_view.php

<?php

namespace cx_newsletter;

class manage_messages_view 
extends view
implements view_interface {
    public function connect() : void {
        \add_submenu_page(
            'cx_newsletter',
            __('Manage messages', 'cx_newsletter'),
            __('Manage messages', 'cx_newsletter'),
            'customize',
            'cx_newsletter_manage_messages',
            [$this, 'action']
        );
    }

    public function process() : void {
        $this->add_activity(manage_messages_activity::get_name());
        $this->add_activity(edit_message_activity::get_name());
        $this->add_activity(send_message_activity::get_name());
    }   
}