When a Worker is soft-deleted, references to it are not cleaned up by
the database (that only happens when it is really deleted). As a result,
the "last-assigned worker of a task" field is still set, but the worker
itself cannot be fetched any more.
This is just a quick fix to avoid an error. It's probably better to
remove the soft-deletion of Workers, as the feature is not really used
anyway. Or to implement it properly, so that the info is used.
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