Add a Custom email notification to WC actions

I’ve followed some tutorials and thread here, But I’ve surely missed something.

Here is the code used (I’ve removed all which is not concerned):

<?php

if ( ! defined( 'ABSPATH' ) ) {
    exit;
}

if ( ! class_exists( 'WC_Email' ) ) {
    return;
}

/**
 * Class WC_Customer_Failed_Order
 */
class WC_Customer_Invoice_Order extends WC_Email {

    public $resp2;

    /**
     * Create an instance of the class.
     *
     * @access public
     * @return void
     */
    public function __construct() {

        // Email slug we can use to filter other data.
        $this->id          = 'wc_customer_invoice_order';
        $this->title       = __( 'An invoice is available', 'textdomain' );
        $this->description = __( 'An e-mail is sent when an invoice is left on an order.', 'textdomain' );
        // For admin area to let the user know we are sending this email to customers.
        $this->customer_email = true;
        $this->heading        = __( 'An invoice is available', 'textdomain' );
        // translators: placeholder is {blogname}, a variable that will be substituted when email is sent out.
        $this->subject = sprintf( _x( '[%s] Invoice available', 'Objet par défaut pour les emails des commandes échouées destinés aux clients', 'textdomain' ), '{blogname}' );

        // Template paths.
        $this->template_html  = 'emails/wc-customer-notice-invoice-order.php';
        $this->template_plain = 'emails/plain/wc-customer-notice-invoice-order.php';
        $this->template_base  = PLUGIN_PATH . 'templates/';

        // Manual email notification
        add_action( 'woocommerce_order_actions', array( $this, 'wc_add_order_meta_box_action' ) );
        add_action( 'woocommerce_order_action_send_invoice_notification', array( $this, 'trigger' ), 10, 2 );

        parent::__construct();
    }

    /**
     * Get content html.
     *
     * @access public
     * @return string
     */
    public function get_content_html() {
        return wc_get_template_html(
            $this->template_html,
            array(
                'order'              => $this->object,
                'email_heading'      => $this->get_heading(),
                'additional_content' => $this->get_additional_content(),
                'sent_to_admin'      => false,
                'plain_text'         => false,
                'email'              => $this,
            ),
            '',
            $this->template_base
        );
    }

    /**
     * Get content plain.
     *
     * @return string
     */
    public function get_content_plain() {
        return wc_get_template_html(
            $this->template_plain,
            array(
                'order'              => $this->object,
                'email_heading'      => $this->get_heading(),
                'additional_content' => $this->get_additional_content(),
                'sent_to_admin'      => false,
                'plain_text'         => true,
                'email'              => $this,
            ),
            '',
            $this->template_base
        );
    }

    //https://gist.github.com/bekarice/5233ed58c3a836064123b290463241c0
    public function wc_add_order_meta_box_action( $actions ) {

        // add "custom notif" custom action
        $actions['send_invoice_notification'] = __( 'Send notification', 'textdomain' );
        return $actions;
    }

    public function trigger( $order_id, $order ){
        error_log( sprintf( 'email triggered %s', $order->get_id() ) );
        if ( ! empty( $_POST['acf']['field_61ab6dd460813'] ) ) {
            foreach ( $_POST['acf']['field_61ab6dd460813'] as $key => $value ) {
                $already_sent = get_post_meta( $order_id, $key, true );
                if ( empty( $already_sent ) ) {
                    $this->object = wc_get_order( $order_id );

                    if ( version_compare( '3.0.0', WC()->version, '>' ) ) {
                        $order_email = $this->object->billing_email;
                    } else {
                        $order_email = $this->object->get_billing_email();
                    }

                    $this->recipient = $order_email;

                    if ( ! $this->is_enabled() || ! $this->get_recipient() ) {
                        return;
                    }

                    $return = $this->send( $this->get_recipient(), $this->get_subject(), $this->get_content(), $this->get_headers(), $this->get_attachments() );

                    if ( $return ) {
                        update_post_meta( $order_id, $key, '1' );
                    }
                }
            }
        }
        return false;

    }

}

and here the plugin main file:

<?php

/**
 * Plugin Name: MyPlugin
 **/

if ( ! defined( 'ABSPATH' ) ) {
    exit;
} // Exit if accessed directly.

class myClass {


    public function __construct() {

        add_filter( 'woocommerce_email_classes', array( $this, 'register_email' ) );

    }



    /**
     * @param array $emails
     *
     * @return array
     */
    public function register_email( $emails ) {
        require 'inc/class-wc-customer-notice-invoice-order.php';
        $emails['WC_Customer_Invoice_Order'] = new WC_Customer_Invoice_Order();

        return $emails;
    }

}

new myClass();

When I try to send a notification, the hook "woocommerce_order_action_send_invoice_notification" is well played but I got an error "WC_Email" class not found.

Do you have any idea what’s wrong ?