Otherwise this will lead to a corrupt state:
ERROR: Session: XMLNode describing a AudioRegion references an unknown source id
ERROR: Session: cannot create Region from XML description. Can not load state for region
ERROR: Playlist: cannot create region from XML
and a track without playlist is created, resulting in a later crash.
Eventually SessionPlaylists::load() needs to handle this gracefully,
but this should help catch cases causing the actual issue.
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