"AWT-EventQueue-0" java.lang.NullPointerException and JInternalFrame

Question

I have two classes one with main method second with GUI methods (JFrame, JInternalFrame). When I call method to start second JInternalFrame from main everything is working but if i call it form any other method i get:
Exception in thread "main" java.lang.NullPointerException
at pkg1.GUI.createFrame(GUI.java:123)
at pkg1.GUI.startFrame2(GUI.java:66)
at pkg1.Top.cos(Top.java:25)
at pkg1.Top.main(Top.java:20) My code:
GUI class
package pkg1;
import java.awt.Component;
import java.awt.Dimension;
import java.awt.Frame;
import java.awt.Toolkit;
import java.awt.event.MouseMotionListener;
import javax.swing.JDesktopPane;
import javax.swing.JFrame;
import javax.swing.JInternalFrame;
import javax.swing.JPanel;
import javax.swing.plaf.basic.BasicInternalFrameUI;
import oracle.jdeveloper.layout.XYLayout;
public class GUI
    public JDesktopPane desktop;
    private XYLayout xYLayout1 = new XYLayout();
    public int openFrameCount = 0;
    JFrame f = new JFrame();
    public void GUII()  // Prepare JFrame
        f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        f.setSize(500, 600);
        f.setVisible(true);
    public void startFrame()
        desktop = new JDesktopPane();
        createFrame(); //create first "window"
        f.setContentPane(desktop);
        desktop.setDragMode(JDesktopPane.OUTLINE_DRAG_MODE);
    public void startFrame2()
        createFrame(); //create second "window"
        f.setContentPane(desktop);
        desktop.setDragMode(JDesktopPane.OUTLINE_DRAG_MODE);
    public void createFrame()
        MyInternalFrame frame = new MyInternalFrame();
        frame.setVisible(true); //necessary as of 1.3
        desktop.add(frame);
        frame.add(new GUI2());
    } Top class
public class Top
    public static void main(String[] args)
        GUI g = new GUI();
        g.GUII(); //Create JFrame
        g.startFrame(); //Create JInternalFrame
        Top t = new Top();
        t.sth();
    public void sth()
        GUI gui = new GUI();
        gui.startFrame2();
} MyIntternalFrame class
import javax.swing.JInternalFrame;
import java.awt.event.*;
import java.awt.*;
import javax.swing.BorderFactory;
import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JPanel;
import javax.swing.SwingConstants;
import oracle.jdeveloper.layout.XYConstraints;
import oracle.jdeveloper.layout.XYLayout;
/* Used by InternalFrameDemo.java. */
public class MyInternalFrame extends JInternalFrame {
    static int openFrameCount =  0;      
    static final int xOffset = 30, yOffset = 30;
    private XYLayout xYLayout1 = new XYLayout();
    private JButton jButton1 = new JButton();
    private JLabel jLabel1 = new JLabel();
    private JFrame c = new JFrame();
    private JPanel d = new JPanel();
    private XYLayout xYLayout2 = new XYLayout();
    public MyInternalFrame() {         
        super("Document #"  + (++openFrameCount),true /*resizable*/,true /*closable*/,true /*maximizable*/,true);//iconifiable*/
        //...Create the GUI and put it in the window...
        //...Then set the window size or call pack...
        int width = new GUI2().width + 10;
        int height = new GUI2().height + 40;
        setSize(width,height);
        setLocation(xOffset*openFrameCount, yOffset*openFrameCount);
} Please tel me where is my mistake or maybe you knew another way to open JInternalFrame with public method form another class

Answer 1

Some possibly helpful suggestions:
1) Create one JDesktopPane, and do it in the constructor of GUI. You should call this constructor only once.
2) Get rid of GUII. The GUI constructor will do all this and more.
3) Get rid of startFrame and startFrame2.
4) In GUI2, change your width and height to static variables if you are going to use them in outside classes. There is no reason to have to create a GUI2 object just to get those values. Get them from the class, not the instances.
5) You're doing something funky in your Top class with your mixing of static code and non-static code. My gut tells me that Top is just a static universe that exists to get your code up and running, and that the code within it should all be static, but that's just my personal opinion.
6) In MyInternalFrame, get the height and width from GUI2 (if that's what you want to do) again in a static fashion. Rather than new GUI2().width which makes no sense, use GUI2.width.
Why can't you put the button inside of the JInternalFrame object? I believe that the contentPane of this object which may hold your button (unless you embed a jpanel) uses BorderLayout, so you have to take care how you add jcomponents to the jinternalframe (or more precisely, it's contentPane).