Why does my JPanel not show in JFrame after button is clicked.
There's my code:

```
import java.awt.EventQueue;

import javax.swing.JFrame;
import javax.swing.JList;

import javax.swing.DefaultListModel;
import javax.swing.AbstractListModel;
import javax.swing.JTable;
import javax.swing.table.DefaultTableModel;
import javax.swing.JButton;
import java.awt.event.MouseAdapter;
import java.awt.event.MouseEvent;
import javax.swing.JPanel;

public class Main {

    private JFrame frame;
    private JTable table;

    /**
     * Launch the application.
     */
    public static void main(String[] args) {
        EventQueue.invokeLater(new Runnable() {
            public void run() {
                try {
                    Main window = new Main();
                    window.frame.setVisible(true);
                } catch (Exception e) {
                    e.printStackTrace();
                }
            }
        });
    }

    /**
     * Create the application.
     */
    public Main() {
        initialize();
    }

    /**
     * Initialize the contents of the frame.
     */
    private void initialize() {
        frame = new JFrame();
        frame.setResizable(false);
        frame.setBounds(100, 100, 450, 300);
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.getContentPane().setLayout(null);

        table = new JTable();
        table.setModel(new DefaultTableModel(
            new Object[][] {
                {"1", "Marcin Zelek", "537573656"},
                {"2", "Krzysztof Tomala", "324159103"},
                {"3", "Zbigniew S", "324159104"},
            },
            new String[] {
                "#", "Name", "Phone number"
            }
        ));
        table.getColumnModel().getColumn(1).setPreferredWidth(214);
        table.getColumnModel().getColumn(2).setPreferredWidth(246);
        table.setBounds(12, 103, 426, 185);
        frame.getContentPane().add(table);

        JButton btnDodajNowy = new JButton("Dodaj nowy");
        btnDodajNowy.addMouseListener(new MouseAdapter() {
            @Override
            public void mouseReleased(MouseEvent arg0) {
                JPanel panel = new JPanel(null);// Creating the JPanel
                panel.setBounds(0, 243, 286, 150);
                panel.setVisible(true);

                JButton button = new JButton("New button");
                button.setBounds(12, 12, 117, 25);
                panel.add(button);

                frame.getContentPane().add(panel);
            }
        });
        btnDodajNowy.setBounds(12, 30, 117, 25);
        frame.getContentPane().add(btnDodajNowy);

        JButton btnUsuZaznaczone = new JButton("Usuń zaznaczone");
        btnUsuZaznaczone.addMouseListener(new MouseAdapter() {
            @Override
            public void mouseReleased(MouseEvent arg0) {
                DefaultTableModel model = (DefaultTableModel) table.getModel();
                int[] selection = table.getSelectedRows();
                for (int i = 0; i < selection.length; i++)
                {
                    model.removeRow(selection[i]-i);
                }
            }
        });
        btnUsuZaznaczone.setBounds(141, 30, 204, 25);
        frame.getContentPane().add(btnUsuZaznaczone);

    }
}

```

Thanks.