How do i fix this error even though there is no red text

Right now i'm making a GUI and i'm getting this error code

Exception in thread "main" java.awt.IllegalComponentStateException: contentPane cannot be set to null.
	at java.desktop/javax.swing.JRootPane.setContentPane(JRootPane.java:594)
	at java.desktop/javax.swing.JFrame.setContentPane(JFrame.java:679)
	at GUI.main(GUI.java:13)

here is my base code

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

public class GUI {

    private Container mainPanel;

    public static void main(String[] args) {
        JFrame frame = new JFrame("Loot and Scoot");
        frame.setContentPane(new GUI().mainPanel);
        frame.setPreferredSize(new Dimension(1280,700));
        frame.pack();
        frame.setVisible(true);
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setLayout(new BorderLayout());
    }
}

I've tried fixing this problem with Alt+Enter but nothing is wrong there is no red text

All you need to do is add a

mainPanel = new JPanel

i've edited my code correctly so it works now, here it is

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

public class GUI {

    private static Container mainPanel;

    public static void main(String[] args) {

        mainPanel = new JPanel();

        JFrame frame = new JFrame("Loot and Scoot");
        frame.setContentPane(new GUI().mainPanel);
        frame.setPreferredSize(new Dimension(1280,700));
        frame.pack();
        frame.setVisible(true);
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setLayout(new BorderLayout());
    }
}

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

public class GUI extends JPanel{
  
    public static void main(String[] args) {

        mainPanel = new JPanel();

        JFrame frame = new JFrame("Loot and Scoot");
        frame.setContentPane(new GUI());
        frame.setPreferredSize(new Dimension(1280,700));
        frame.pack();
        frame.setVisible(true);
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setLayout(new BorderLayout());
    }
}