Notification error: OrderRepository::getOrderByOrderNumber() must be of the type string, null given...

[gychem]

Describe the bug
Recently we're getting more alerts of this error triggered in the Adyen shopware6 extension:

Error: Argument 1 passed to Adyen\Shopware\Service\Repository\OrderRepository::getOrderByOrderNumber() must be of the type string, null given, called in /var/www/rsca/shopware/custom/plugins/AdyenPaymentShopware6/src/ScheduledTask/ProcessNotificationsHandler.php on line 363 in /var/www/rsca/shopware/custom/plugins/AdyenPaymentShopware6/src/Service/Repository/OrderRepository.php on line 91

What is causing this error?

Versions
Shopware version: 6.4.20.1
Plugin version: 3.11.0

To Reproduce
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Expected behavior
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Screenshots
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Desktop (please complete the following information):
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Smartphone (please complete the following information):
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Additional context
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