Sorry forget my answer before...  
Change this: 
if (@Mail::Send(
                            (int) $order->id_lang,
                            'in_transit',
                            Mail::l('Package in transit', (int) $order->id_lang),
                            $templateVars,
                            $customer->email,
                            $customer->firstname.' '.$customer->lastname,
                            null,
                            null,
                            null,
                            null,
                            _PS_MAIL_DIR_,
                            true,
                            (int) $order->id_shop
                        )) {
                            Hook::exec('actionAdminOrdersTrackingNumberUpdate', ['order' => $order, 'customer' => $customer, 'carrier' => $carrier], null, false, true, false, $order->id_shop);
                            Tools::redirectAdmin(static::$currentIndex.'&id_order='.$order->id.'&vieworder&conf=4&token='.$this->token);
                        } else {
                            $this->errors[] = Tools::displayError('An error occurred while sending an email to the customer.');
                        } 
To:
Hook::exec('actionAdminOrdersTrackingNumberUpdate', ['order' => $order, 'customer' => $customer, 'carrier' => $carrier], null, false, true, false, $order->id_shop);
                            Tools::redirectAdmin(static::$currentIndex.'&id_order='.$order->id.'&vieworder&conf=4&token='.$this->token); 
I have tried this solution. So please test it out!