Javagui NullPointerException error.

package music_game_1;

import java.awt.Color;
import java.awt.Graphics;
import java.awt.Image;
import java.awt.event.MouseAdapter;
import java.awt.event.MouseEvent;
import java.awt.event.MouseMotionAdapter;

import javax.swing.ImageIcon;
import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JLabel;

public class Musicgame extends JFrame {

    private Image screen_Image;
    private Graphics screen_Graphic;

    private ImageIcon backimage = new ImageIcon(Main.class.getResource("../Images/backimage.png"));
    private ImageIcon backimagebasic = new ImageIcon(Main.class.getResource("../Images/backimgaebasic.png"));

    public Musicgame() {
        setUndecorated(true);
        setTitle("Musicgame");
        setSize(Main.SCREEN_WIdTH, Main.SCREEN_HEIGH);
        setResizable(false);
        setLocationRelativeTo(null);
        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        setVisible(true); 
        setBackground(new Color(0, 0, 0, 0));
        setLayout(null);

        backButton.setBounds(1895, 0, 30, 30);
        backButton.setBorderPainted(false);
        backButton.setContentAreaFilled(false);
        backButton.setFocusPainted(false);
        backButton.addMouseListener(new MouseAdapter() {
            @Override
            public void mouseEntered(MouseEvent e) {
                backButton.setIcon(backimage);
            }

            @Override
            public void mouseExited(MouseEvent e) {
                backButton.setIcon(backimagebasic);
            }

            @Override
            public void mousePressed(MouseEvent e) {
                System.exit(0);
            }
        });
        add(backButton);
    ```
    I'm making a rhythm game. Exception in thread "main" java.lang.NullPointerException
    at javax.swing.ImageIcon.<init>(Unknown Source)
    at music_game_1.Musicgame.<init>(Musicgame.java:21)
    at music_game_1.Main.main(Main.java:10)
I got this error, but when I looked it up,
- Causes: Occurs when you try to use a null object without creating an instance after you create an object to use. I don't know how to fix it. Is there anyone who can solve this?"T"