Error "Notice: Undefined offset: 0" while acting any email template action (opening or sending test email)

[mmc-edizioni]

I'm getting this error while trying to execute any of the action available for email templates (Design -> Email Theme -> any of three dots menu action)

Symfony\Component\Debug\Exception\ ContextErrorException
in src/Adapter/MailTemplate/MailPreviewVariablesBuilder.php (line 173)

         * @throws \SmartyException
         */
        private function buildOrderVariables(LayoutInterface $mailLayout)
        {
            $orders = Order::getOrdersWithInformations(1);
            $order = new Order($orders[0]['id_order']);
            if (self::ORDER_CONFIRMATION == $mailLayout->getName()) {
                $productTemplateList = $this->getProductList($order);
                $productListTxt = $this->mailPartialTemplateRenderer->render('order_conf_product_list.txt', $this->context->language, $productTemplateList);
                $productListHtml = $this->mailPartialTemplateRenderer->render('order_conf_product_list.tpl', $this->context->language, $productTemplateList);

MailPreviewVariablesBuilder->buildOrderVariables() in src/Adapter/MailTemplate/MailPreviewVariablesBuilder.php (line 145)
MailPreviewVariablesBuilder->buildTemplateVariables() in src/PrestaShopBundle/Controller/Admin/Improve/Design/MailThemeController.php (line 479)
MailThemeController->renderLayout() in src/PrestaShopBundle/Controller/Admin/Improve/Design/MailThemeController.php (line 416)
MailThemeController->previewLayoutAction() in vendor/symfony/symfony/src/Symfony/Component/HttpKernel/HttpKernel.php (line 151)
HttpKernel->handleRaw() in vendor/symfony/symfony/src/Symfony/Component/HttpKernel/HttpKernel.php (line 68)
HttpKernel->handle() in vendor/symfony/symfony/src/Symfony/Component/HttpKernel/Kernel.php (line 200)
Kernel->handle() in admin541fbmqny/index.php (line 82) 

So I'm currently not able to modify it and/or send test email....

Is there a way to fix it? I tried to look for this error in forum, but I've been not able to find something related to.

Hereafter the site info data:
 

--Informazioni sul server

Informazioni sul server: Linux #1 SMP Tue Jun 28 15:37:28 UTC 2022 x86_64

Versione software del server: Apache/2.4.6

Versione di PHP: 7.4.32

Limite di memoria: 256M

Tempo di esecuzione massimo: 600

Dimensione Max. dei file da caricare: 1G

-- Informazioni datatbase

Versione MySQL: 10.1.48-MariaDB

Server MySQL: localhost

Motore MySQL: InnoDB

Driver MySQL: DbPDO

-- Elenco delle sostituzioni

-- Informazioni negozio

Versione di PrestaShop: 1.7.8.7

Tema attualmente utilizzato: classic

-- Informazioni sulla configurazione mail

Metodo invio mail: Stai utilizzando /usr/sbin/sendmail

-- I tuoi dati

Il tuo browser: Mozilla/5.0 (X11; Linux x86_64; rv:105.0) Gecko/20100101 Firefox/105.0

-- Verifica la tua configurazione

Parametri richiesti: OK

Parametri opzionali: OK

Thank you in advance for any hint/help
Regards