Calling outer class' method getContentPane() from inner Action class

I want to call outer class' method getContentPane() from inner Action class. I don't understand why my code doesn't work.

public class MainFrame extends JFrame {
    public MainFrame() {
        super("My app");
        JMenuBar menuBar = new JMenuBar();
        setJMenuBar(menuBar);

        JMenu myMenu = new JMenu("File");
        menuBar.add(myMenu);
        Action myAction = new AbstractAction("Do everything") {
            public void actionPerformed(ActionEvent e) {
                JPanel panel = new JPanel();
                panel.setBackground(Color.CYAN);
                getContentPane().add(panel, BorderLayout.CENTER);
            }
        };
        myMenu.add(myAction);
    }
}

You must call validate(); method after getContentPane().add(...);.

The validate method is used to cause a container to lay out its subcomponents again. It should be invoked when this container's subcomponents are modified (added to or removed from the container, or layout-related information changed) after the container has been displayed.