Could not send email to <...> Unable to open path to message files

[megmed]

Sometimes I get the following error in the message queue list of the notification center:
Fehler beim Senden der Nachricht. Prüfen Sie das System-Log für Details.

In the contao system log the following ist stated:
Could not send email to "[email protected]" for message ID 11: Unable to open path "test/contao4/var/notification_center/1312/Attachment.pdf"

I assume that this happens when the message queue tries to resend an email that could not be sent on the first attempt. I had a quick look in the source code and it seems that the message files are deleted no matter the email could be sent successfully or not.

contao-notification_center/library/NotificationCenter/Queue/QueueManager.php

Lines 100 to 110 in bd6f6e4

	foreach ($messages as $msg) {
	/* @var $msg QueuedMessage */
	$result = $msg->send();
	if (!$result) {
	$msg->error = 1;
	} else {
	$msg->dateSent = time();
	}
	// Remove the temporary message files
	$this->removeMessageFiles($msg->id);

I guess $this->removeMessageFiles($msg->id); should only be called if the email was sent successfully?
Like this:

if (!$result) { 
         $msg->error = 1; 
     } else { 
         $msg->dateSent = time(); 
         // Remove the temporary message files 
         $this->removeMessageFiles($msg->id);
     } 

After that, adding the message to the queue again using the button from the queue list should also work, which is currently not the case for messages with file attachments.