How to use SystemTray correctly in Java?

Question

I tried to implement a small (test) programm using the SystemTray in java. I got problems and wrote a (M)WE and pubushed it with http://pastebin.com/nYBkaLSy (sorry for not doing any safty checks).

What happens (under Ubuntu/KDE with openJDK):

• The program starts and the button is clickable. The SystemTray is registered (all right so far)
• If I click (single left mouse) on the image in the tray the button is no more clickable. Instead it seems to be disabled. A right-click on the tray icon OR the frame content (!!!) opens the popup. After closing this (clicking anywhere) the button active again. (this bahavior I cannot explain. My code does not implie this as far as I see)
• If I click on the minimize button of the window manager the frame disappears (correct)
• On choosing show from the popup of the tray icon restores the frame and the frame content is clickable (ok so far).
• Unfortunately the popup is disabled after that. I cannot open it again and if I minimize the window again I cannot bring the window up again. (also I do not see the point in the code)

Now I am unsure if I understood the principles wrongly or if there is some other bug around. SO please help me to clear my questions.

EDIT: I inserted the code (a bit modifie) here.

Please note that the created image is just dummy code. In my implementation I load an external image (see comments) which produces the same result. So the iconToImage() hint in the comments seems not to be a problem.

import java.awt.AWTException;
import java.awt.Image;
import java.awt.MenuItem;
import java.awt.PopupMenu;
import java.awt.SystemTray;
import java.awt.TrayIcon;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.awt.event.WindowEvent;
import java.awt.event.WindowStateListener;
import java.awt.image.BufferedImage;
import java.net.URL;

import javax.swing.ImageIcon;
import javax.swing.JButton;
import javax.swing.JFrame;

public class TestTray {

    public static void main(String[] args) {
        // Create tmp image
        Image image = new BufferedImage(32, 32, BufferedImage.TYPE_INT_RGB);
        image.getGraphics().drawOval(2, 2, 30, 30);

        // Alternative: Load PNG image
        //URL imUrl = TestTray.class.getResource("clock.png");
        //ImageIcon icon = new ImageIcon(imUrl);
        //image = icon.getImage();

        TrayIcon ti = new TrayIcon(image);
        ti.setImageAutoSize(true);
        try {
            SystemTray.getSystemTray().add(ti);
        } catch (AWTException e) {
            e.printStackTrace();
        }

        // Create JFrame and set default things
        final JFrame frame = new JFrame();
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.add(new JButton("Test-Button"));
        frame.pack();
        frame.setVisible(true);

        // Add listener to hide window in case of minimization
        frame.addWindowStateListener(new WindowStateListener() {
            @Override
            public void windowStateChanged(WindowEvent ev) {
                if(ev.getNewState() == JFrame.ICONIFIED)
                    frame.setVisible(false);
            }
        });

        // Create popup for System tray and register it
        PopupMenu popup = new PopupMenu();
        MenuItem menuItem;

        menuItem = new MenuItem("Show");
        menuItem.addActionListener(new ActionListener() {
            @Override
            public void actionPerformed(ActionEvent e) {
                frame.setVisible(true);
                frame.setExtendedState(JFrame.NORMAL);
            }
        });
        popup.add(menuItem);

        menuItem = new MenuItem("Exit");
        menuItem.addActionListener(new ActionListener() {
            @Override
            public void actionPerformed(ActionEvent arg0) {
                System.exit(0);
            }
        });
        popup.add(menuItem);

        ti.setPopupMenu(popup);
    }

}