WorldWind Layer manager panel is blank

Question

I am developing a WorldWind application and I want to reuse the LayerAndElevationManagerPanel example class in my own application. I am trying to put it into a dialog box that the user can pull up on demand, however when the dialog is displayed the scroll pane for the panel is empty. Can this example be reused?

Here is the example code from WorldWind

/*
 * Copyright (C) 2013 United States Government as represented by the Administrator of the
 * National Aeronautics and Space Administration.
 * All Rights Reserved.
 */

package gov.nasa.worldwindx.examples.layermanager;

import gov.nasa.worldwind.WorldWindow;

import javax.swing.*;
import java.awt.*;

/**
 * Combines the layer manager and elevation model manager in a single frame.
 *
 * @author tag
 * @version $Id$
 */
public class LayerAndElevationManagerPanel extends JPanel
{
    protected LayerManagerPanel layerManagerPanel;
    protected ElevationModelManagerPanel elevationModelManagerPanel;

    public LayerAndElevationManagerPanel(WorldWindow wwd)
    {
        super(new BorderLayout(10, 10));

        this.add(this.layerManagerPanel = new LayerManagerPanel(wwd), BorderLayout.CENTER);

        this.add(this.elevationModelManagerPanel = new ElevationModelManagerPanel(wwd), BorderLayout.SOUTH);
    }

    public void updateLayers(WorldWindow wwd)
    {
        this.layerManagerPanel.update(wwd);
    }

    public void updateElevations(WorldWindow wwd)
    {
        this.elevationModelManagerPanel.update(wwd);
    }

    public void update(WorldWindow wwd)
    {
        this.updateLayers(wwd);
        this.updateElevations(wwd);
    }
}

Here is my code

import java.awt.BorderLayout;
import java.awt.FlowLayout;

import javax.swing.GroupLayout;
import javax.swing.GroupLayout.Alignment;
import javax.swing.JButton;
import javax.swing.JDialog;
import javax.swing.JPanel;
import javax.swing.JScrollPane;
import javax.swing.border.EmptyBorder;

import gov.nasa.worldwind.WorldWindow;
import gov.nasa.worldwindx.examples.layermanager.LayerAndElevationManagerPanel;

public class LayerDialog extends JDialog
{

    private final JPanel contentPanel = new JPanel();

    /**
     * Create the dialog.
     */
    public LayerDialog(WorldWindow wwd)
    {
        setTitle("Layer Manager");
        setBounds(100, 100, 386, 360);
        getContentPane().setLayout(new BorderLayout());
        contentPanel.setBorder(new EmptyBorder(5, 5, 5, 5));
        getContentPane().add(contentPanel, BorderLayout.CENTER);
        JScrollPane scrollPane = new JScrollPane();
        scrollPane.setViewportBorder(null);
        scrollPane.add(new LayerAndElevationManagerPanel(wwd));
        GroupLayout gl_contentPanel = new GroupLayout(contentPanel);
        gl_contentPanel.setHorizontalGroup(
            gl_contentPanel.createParallelGroup(Alignment.LEADING)
                .addGroup(gl_contentPanel.createSequentialGroup()
                    .addContainerGap()
                    .addComponent(scrollPane, GroupLayout.PREFERRED_SIZE, 344, GroupLayout.PREFERRED_SIZE)
                    .addContainerGap(6, Short.MAX_VALUE))
        );
        gl_contentPanel.setVerticalGroup(
            gl_contentPanel.createParallelGroup(Alignment.LEADING)
                .addGroup(gl_contentPanel.createSequentialGroup()
                    .addContainerGap()
                    .addComponent(scrollPane, GroupLayout.DEFAULT_SIZE, 257, Short.MAX_VALUE)
                    .addContainerGap())
        );
        contentPanel.setLayout(gl_contentPanel);
        {
            JPanel buttonPane = new JPanel();
            buttonPane.setLayout(new FlowLayout(FlowLayout.RIGHT));
            getContentPane().add(buttonPane, BorderLayout.SOUTH);
            {
                JButton okButton = new JButton("OK");
                okButton.setActionCommand("OK");
                buttonPane.add(okButton);
                getRootPane().setDefaultButton(okButton);
            }
            {
                JButton cancelButton = new JButton("Cancel");
                cancelButton.setActionCommand("Cancel");
                buttonPane.add(cancelButton);
            }
        }
    }

}

Here is an excerpt from MainWindow

    public MainWindow(App a)
    {

       WorldWindowGLCanvas wwd = new WorldWindowGLCanvas();

       JMenuItem mntmLayers = new JMenuItem("Layers");
       mntmLayers.addActionListener(new ActionListener()
       {
           @Override
           public void actionPerformed(ActionEvent e)
          {
               try
               {
                   LayerDialog dialog = new LayerDialog(wwd);
                   dialog.setDefaultCloseOperation(JDialog.DISPOSE_ON_CLOSE);
                   dialog.setVisible(true);
               }
               catch (Exception ex)
               {
                   ex.printStackTrace();
               }
           }
       });
    }