Problems with AbstractPortObject from 2.x used in 3.1

Hi, I'm developing with knime-2 api and derived my port object from AbstractPortObject:

public class SeesarPortObject extends AbstractPortObject {

    public static final PortType TYPE = new PortType(SeesarPortObject.class);
    public static final PortType TYPE_OPTIONAL = new PortType(SeesarPortObject.class, true);
    :

Everything works fine in knime-2.x, but in knime-3.1, when trying to save an executed workflow with a node that has such a port type as output, I get the following error in knime.log:

2016-06-22 09:32:18,384 : DEBUG : ModalContext : SaveWorkflowRunnable :  :  : Could not save workflow
java.util.NoSuchElementException: No value present
	at java.util.Optional.get(Optional.java:135)
	at org.knime.core.node.FileNodePersistor.savePortObject(FileNodePersistor.java:1425)
	at org.knime.core.node.FileNodePersistor.savePort(FileNodePersistor.java:1389)
        :

I have seen that built-in port objects like ImagePortObject were updated for knime-3 api to use the PortTypeRegistry:

public static final PortType TYPE = PortTypeRegistry.getInstance().getPortType(ImagePortObject.class);

And, when looking into the PortTypeRegistry's method getObjectSerializer(), there is a special legacy handling to deal with knime-2 PortTypes, and there another special treatment of AbstractSimplePortObject, but no special handling of AbstractPortObject.

My question: Has anybody else got similar problems with nodes using AbstractPortObject developed in knime-2 and using in knime-3?

Best regards, Frank