Magento2 | Change subject email of new order if condition is true

I created an observer that changes the email subject when I place a new order:
This is the ChangeOrderEmailSubject.php file:

<?php
namespace SalesOrderCustObserver;

use MagentoFrameworkEventObserverInterface;
use MagentoFrameworkEventObserver;
use MagentoSalesModelOrder;

class ChangeOrderEmailSubject implements ObserverInterface
{
    public function execute(Observer $observer)
    {
        /** @var Order $order */
        $order = $observer->getEvent()->getOrder();

        $orderNumber = $order->getIncrementId();
        $customValue = $order->getIdreorder();

        $emailSubject = __("Order N.") . $orderNumber;

        if ($customValue != "") {
            $emailSubject .= " Riordino N.".$customValue;
        }

        /** @var MagentoSalesModelOrderEmailContainer $emailContainer */
        $emailContainer = $observer->getEvent()->getContainer();
        $emailContainer->setSubject($emailSubject);
    }
}

Instead, this is the events.xml file:

<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:Event/etc/events.xsd">
    <event name="sales_order_place_after">
        <observer name="custom_shipping_description" instance="SalesOrderCustObserverCustomShippingDescriptionObserver" method="execute" />
        <observer name="change_order_email_subject" instance="SalesOrderCustObserverChangeOrderEmailSubject" />
    </event>
</config>

Unfortunately when I try to execute the new order this is the error code I get “main.CRITICAL: Error: Call to a member function setSubject() on null in ”. How can I solve this problem? Maybe “MagentoSalesModelOrderEmailContainer” is not the correct method to call to change the subject of the new order email?