public class DefaultFlushEntityEventListener implements FlushEntityEventListener {
// omitted code
    private Object getNextVersion(FlushEntityEvent event) throws HibernateException {
         // omitted code
        // need to check for nul before call this entry.getVersion()
        final Object nextVersion = isVersionIncrementRequired ?
		 Versioning.increment( entry.getVersion(), persister.getVersionType(), event.getSession() ) :
                 entry.getVersion(); //use the current version
        }
// omitted code
}

public class DefaultFlushEntityEventListener implements FlushEntityEventListener {
// omitted code

    // TODO Improve exception message
    private void checkVersion(EntityPersister persister, EntityEntry entry){
        if(entry.getVersion() == null){
            throw new HibernateException(
                String.format("null version number in entity [%s] with identifier %s", 
                persister.getEntityName(), entry.getId()));
        }
    }

    private Object getNextVersion(FlushEntityEvent event) throws HibernateException {
         // omitted code
 
         // TODO merge those two 'if'
        if(isVersionIncrementRequired && entry.getVersion() == null){
            checkVersion(persister, entry);
        }

        // need to check for nul before call this entry.getVersion()
        final Object nextVersion = isVersionIncrementRequired ?
		Versioning.increment( entry.getVersion(), persister.getVersionType(), event.getSession() ) :
                entry.getVersion(); //use the current version
        }
    // omitted code
}