Как исправить ошибку NullPointerException

Question

Exception in thread "main" java.lang.NullPointerException вы водит при таком коде:

import java.awt.BorderLayout;
import java.awt.FlowLayout;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;

import javax.swing.*;
public class Clicker {
    private static boolean Stage = false;
    private static int Coins = 0;
    private static JButton button4;
    private static JFrame Shop;
    private static JFrame Clicker3;
public static void main (String [] args){
    Clicker3 = new JFrame("Clicker");
    JButton button = new JButton("Click me");
    JButton button2 = new JButton("Shop");
    JPanel panel = new JPanel(new FlowLayout());
    JLabel label = new JLabel("Coins:"+ Coins);
    Clicker3.add(panel, BorderLayout.SOUTH);
    Clicker3.add(label, BorderLayout.NORTH);
    panel.add(button);
    panel.add(button2);
    Clicker3.setVisible(true);
    Clicker3.setSize(350, 350);
    button.addActionListener(new ActionListener(){
        public void actionPerformed(ActionEvent arg0) {
            if(Stage = true){
            Coins=Coins + 1;
            Update();
            }else{
                Coins=Coins +2;
                Update();
            }

        }

        private void Update() {
        label.setText("Coins:"+ Coins);
        }

    });
    button2.addActionListener(new ActionListener(){
        public void actionPerformed(ActionEvent arg1){
            Shop = new JFrame("Shop");
            JPanel panel3 = new JPanel(new FlowLayout());
            button4 = new JButton("X2");
            panel3.add(button4);
            Shop.add(panel3, BorderLayout.CENTER);
            Shop.setVisible(true);
            Shop.setSize(350, 350);
        }
    });
    button4.addActionListener(new ActionListener(){
        public void actionPerformed(ActionEvent arg2){
            if (Coins !=100 | Coins > 100){
                Coins=Coins - 100;
                Stage = true;
                System.out.println("+");
            }else{

                System.out.println("-");
            }
        }
    });

}
}