Error adding/removing AsynchChildren

I have ChildFactory fueled by a Lookup to provide the elements to display.
It works fine, but as I add/remove items from the list I get this type of
error:

SEVERE [org.openide.util.RequestProcessor]: Error in RequestProcessor
org.openide.nodes.AsynchChildren
java.lang.AssertionError: map.size()=3 entries.size()=2

Is there a way to prevent this? I tried avoiding to make the refresh
immediate but didn't help.

Here's the ChildFactory if it helps:

public class PlayerChildFactory extends ChildFactory.Detachable<RPObject>
        implements LookupListener {

    private static final Logger LOG
            = Logger.getLogger(PlayerChildFactory.class.getSimpleName());
    protected final InstanceContent context = new InstanceContent();
    protected final Lookup localLookup = new AbstractLookup(getContext());
    private Lookup.Result<RPObject> result;

    @Override
    protected void addNotify() {
        result = getLocalLookup().lookupResult(RPObject.class);
        result.addLookupListener(this);
    }

    @Override
    protected void removeNotify() {
        result.removeLookupListener(this);
        result = null;
    }

    @Override
    protected boolean createKeys(List<RPObject> toPopulate) {
        for (RPObject rs : result.allInstances()) {
            toPopulate.add(rs);
        }
        return true;
    }

    @Override
    protected Node createNodeForKey(RPObject w) {
        try {
            return new PlayerNode(w);
        } catch (IntrospectionException ex) {
            Exceptions.printStackTrace(ex);
            AbstractNode node = new AbstractNode(Children.LEAF);
            node.setDisplayName(Tool.extractName(w));
            return node;
        }
    }

    @Override
    public void resultChanged(LookupEvent ev) {
        refresh(true);
    }

    /**
     * @return the context
     */
    public InstanceContent getContext() {
        return context;
    }

    /**
     * @return the localLookup
     */
    public Lookup getLocalLookup() {
        return localLookup;
    }}

You can see this question in Stackoverflow
<http://stackoverflow.com/questions/36583438/error-adding-removing-asynchchildren>.