使用IntelliJ IDEA GUI Designer在$$$ setupUI $$$上的NullPointer异常

Question

I've been trying to use a GUI I made in IntelliJ IDEAS GUI Designer in an applet I'm working on. I've gotten the thing to run when I just have one label inside my root JPanel but for some reason when I add more components I get the following error:

java.lang.NullPointerException
at java.awt.Container.addImpl(Container.java:1095)
at java.awt.Container.add(Container.java:971)
at inputGui.$$$setupUI$$$(inputGui.java)
at inputGui.<init>(inputGui.java:25)
at HelloWorld.init(HelloWorld.java:11)
at sun.applet.AppletPanel.run(AppletPanel.java:435)
at java.lang.Thread.run(Thread.java:745)

My class for the GUI inputGui.java is laid out like this:

(I will mark the line where it breaks with: //!BROKEN - Line 25! )

public class inputGui extends JFrame {
private JPanel rootNode;
private JTextField id_field;
private JTextField mi_field;
private JTextField lastName_field;
private JTextField address_field;
//more declorations...

public inputGui() {//initialize GUI
        super( "Hello World" );
        setContentPane( rootNode );//!BROKEN - Line 25!
        pack();
        setDefaultCloseOperation( JFrame.EXIT_ON_CLOSE );
        setVisible( false );
    }
    public JPanel getRootNode() {
        return rootNode;
    }
    private void createUIComponents() {
        // TODO: place custom component creation code here
    }
}

And I call it from my Applets 'init()' function:

public class HelloWorld extends JApplet {
    //Called when this applet is loaded into the browser.
    public void init() {
        //Execute a job on the event-dispatching thread; creating this applet's GUI.
        inputGui frame =  new inputGui();//just here to get a clearer error
        try {
            SwingUtilities.invokeAndWait(new Runnable() {
                public void run() {
                    Container content = getContentPane();
                    inputGui frame =  new inputGui();//init GUI
                    content.add(frame.getRootNode() );//add rootNode to JFRame

                }
            });
        } catch (Exception e) {
            System.err.println("createGUI didn't complete successfully");
        }
    }

}

I just cant figure out why adding components suddenly creates a null pointer!. If its any help my elements are nested like this:

Answer 1

Code from inputGui.java

private void createUIComponents() {
   // TODO: place custom component creation code here
}

You selected option "Custom Create" on some component in UI designer. You should create that component by yourself, otherwise it will fail. Uncheck that option and everything should be fine.