See http://mail.openjdk.java.net/pipermail/awt-dev/2011-July/001810.html for details.

Try following a testcase. Upon running, this will bring up a small frame
and a JOptionPane. None of them renders for me, but after some blind
clicking in the option pane it throws exactly this NPE.

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

public class TestRMNPE extends JLabel {
    private TestRMNPE(String l) {
super(l);
    }

    public void paint(Graphics g) {
setText("Frak");
JOptionPane.showMessageDialog(null, "Don't do this");
    }
    
    public static void main(String[] args) {
EventQueue.invokeLater(new Runnable() {
public void run() {
JFrame f = new JFrame();
f.add(new TestRMNPE("Test"));
f.pack();
f.setVisible(true);
}
}
);
    }
}