When try to made a replication form local db4o database to remote db4o database the drs crash and send:

java.lang.NullPointerException
at com.db4o.drs.db4o.FileReplicationProvider.produceReference(Unknown Source)
at com.db4o.drs.inside.InstanceReplicationPreparer.prepareObjectToBeReplicated(Unknown Source)
at com.db4o.drs.inside.InstanceReplicationPreparer.visit(Unknown Source)
at com.db4o.drs.inside.traversal.GenericTraverser.traverseObject(Unknown Source)
at com.db4o.drs.inside.traversal.GenericTraverser.traverseGraph(Unknown Source)
at com.db4o.drs.inside.GenericReplicationSession.prepareGraphToBeReplicated(Unknown Source)
at com.db4o.drs.inside.GenericReplicationSession.replicate(Unknown Source)

The code is:

Db4o.newConfiguration().generateUUIDs(ConfigScope.GLOBALLY);
Db4o.newConfiguration().generateVersionNumbers(ConfigScope.GLOBALLY);
Db4o.newConfiguration().allowVersionUpdates(true);

ObjectContainer local = Db4o.openClient("localhost", 8081, "username","userpassword");
ObjectContainer remote = Db4o.openClient("192.168.1.200", 8081, "username","userpassword");

//Class _class=A propietary class

ReplicationSession replication = Replication.begin(local, remote);
ObjectSet<?> changed = replication.providerA().objectsChangedSinceLastReplication();
if (changed!=null && changed.size()>0){
    while (changed.hasNext()){
        Object obj=changed.next();
if (_class.isInstance(obj)){
replication.replicate(_class.cast(obj)); <------- Here is the exception
}
}
replication.commit();
replication.close();
local.close();
remote.close();