UVDESK Update MailboxService.php (In testing)

[Niijimasama]

I have had a persistent issue with some emails generating tickets but threads are empty. This causes tickets to be un-assignable and unreadable from the UI (the error in DEV mode materializes as a "createdat" null variable when trying to view a ticket). Going through the code revealed that this exception is not handled. I am not a high level programmer so I am still working on this and testing. Goal is to use existing codebase.

1. Why is this change necessary?

2. What does this change do, exactly?

3. Please link to the relevant issues (if any).