saving a coded concept throws exception

In legacy UI (openmrs version 2.1.0 or higher only)-> View concept dictionary -> search for a coded concept -> click on edit and save. It throws a hibernate exception: "A collection with cascade="all-delete-orphan" was no longer referenced by the owning entity instance: org.openmrs.Concept.answers

Here is the stack trace:

rg.hibernate.HibernateException: A collection with cascade="all-delete-orphan" was no longer referenced by the owning entity instance: org.openmrs.Concept.answers
org.hibernate.engine.internal.Collections.processDereferencedCollection(Collections.java:114)
org.hibernate.engine.internal.Collections.processUnreachableCollection(Collections.java:67)
org.hibernate.event.internal.AbstractFlushingEventListener.flushCollections(AbstractFlushingEventListener.java:258)
org.hibernate.event.internal.AbstractFlushingEventListener.flushEverythingToExecutions(AbstractFlushingEventListener.java:103)
org.hibernate.event.internal.DefaultAutoFlushEventListener.onAutoFlush(DefaultAutoFlushEventListener.java:61)