`/api/:category/:id/history` returns status code 500
e.g.
curl https://sshoc-marketplace-api.acdh-dev.oeaw.ac.at/api/tools-services/3IAyEp/history
returns
{"timestamp":"2021-07-23 09:04:40","status":500,"error":"Cannot instantiate abstract class or interface: : eu.sshopencloud.marketplace.model.items.DigitalObject; nested exception is org.hibernate.InstantiationException: Cannot instantiate abstract class or interface: : eu.sshopencloud.marketplace.model.items.DigitalObject"}