Dialog empty with JScrollPane und JPanel

I am trying to add the content to the mainPanel and mainpanel to the mainScrollPane. However, an empty dialog is displayed.

IMPORTANT: it must be implemented with JPanel and JScrollPane...

   `setModal(true);
    setTitle("Edit item");
    setDefaultCloseOperation(DISPOSE_ON_CLOSE);

    JPanel mainPanel = new JPanel();
    mainPanel.setPreferredSize(new Dimension(400, 700));

    JScrollPane mainScrollPane = new JScrollPane();
    mainScrollPane.add(mainPanel);
    setLayout(new BorderLayout());
    add(mainScrollPane);

    mainPanel.add(new JLabel("ID: "));
    mainPanel.add(txtID = new JTextField(item.getID()));
    mainPanel.add(new JLabel("Description: "));
    mainPanel.add(txtDescription = new JTextField(item.getDescription()));


    pack();
    setVisible(true);`

Please provide a minimal reproducible example. It could look like what follows:

MyFrame.java

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

class Item { 
    String id; 
    String description; 
    Item(String id, String description) {this.id = id;this.description = description;}
    String getId() {return id;}
    String getDescription() {return description;}
}

public class MyFrame extends JFrame {
    Item item = new Item("007", "Special watch");
    public MyFrame() {
        // setModal(true);
        setTitle("Edit item");
        setDefaultCloseOperation(DISPOSE_ON_CLOSE);

        JPanel mainPanel = new JPanel();
        mainPanel.setPreferredSize(new Dimension(400, 700));

        mainPanel.add(new JLabel("ID: "));
        mainPanel.add(new JTextField(item.getId()));
        mainPanel.add(new JLabel("Description: "));
        mainPanel.add(new JTextField(item.getDescription()));

        /* From Oracle's documentation, see link below: 
           Creates a JScrollPane that displays the contents of the
           specified component, where both horizontal and vertical
           scrollbars appear whenever the component's contents are
           larger than the view.
        */
        JScrollPane mainScrollPane = new JScrollPane(mainPanel);

        // Just to show the bars
        mainScrollPane.setHorizontalScrollBarPolicy(JScrollPane.HORIZONTAL_SCROLLBAR_ALWAYS);  
        mainScrollPane.setVerticalScrollBarPolicy(JScrollPane.VERTICAL_SCROLLBAR_ALWAYS);  

        getContentPane().add(mainScrollPane); // <- This makes it shine!

        pack();
        setVisible(true);
    }

    public static void main(String[] args) {
        new MyFrame();
    }
}

$ javac MyFrame.java
$ java MyFrame   

Et voilà:

https://docs.oracle.com/javase/8/docs/api/javax/swing/JScrollPane.html