Requesting references inside another one where there is none should throw an error, not crash the server

[PonteIneptique]

See http://dev.chartes.psl.eu/api/nautilus/dts/navigation?id=urn%3Acts%3AfrenchLit%3Apos2001.pos200118.positionThese-fr1&ref=1

Any log available @MrGecko ?