I'm asking you a question because I got an execution error when I pressed the button.

package studyfree;

import java.awt.Button;
import java.awt.Frame;
import java.awt.GridLayout;
import java.awt.Label;
import java.awt.Panel;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.awt.event.WindowEvent;
import java.awt.event.WindowListener;

    class Gugudan extends Frame implements WindowListener, ActionListener{

        @Override
        public void actionPerformed(ActionEvent arg0) {
            Object c = arg0.getSource();
            for (int a=0; a<9; a++) {
                if (c == btns[a]) {
                    for(int b=1;b<10;b++) {
                        lb1.setText(btns[a]+"*"+b); //Error appears here.
                    }
                }
            }
        }
        @Override
        public void windowActivated(WindowEvent e) {
            // // TODO Auto-generated method stub

        }

        @Override
        public void windowClosed(WindowEvent e) {
            // // TODO Auto-generated method stub

        }

        @Override
        public void windowClosing(WindowEvent e) {
            // // TODO Auto-generated method stub
            System.exit(0);
        }

        @Override
        public void windowDeactivated(WindowEvent e) {
            // // TODO Auto-generated method stub

        }

        @Override
        public void windowDeiconified(WindowEvent e) {
            // // TODO Auto-generated method stub

        }

        @Override
        public void windowIconified(WindowEvent e) {
            // // TODO Auto-generated method stub

        }

        @Override
        public void windowOpened(WindowEvent e) {
            // // TODO Auto-generated method stub

        }
        Button [] btns;
        Panel pa1,pa2;
        Label lb1;
        Gugudan(String str){
            super(str);


            pa1 = new Panel();
            pa1.setLayout(new GridLayout(3,3));
            pa1.setSize(500, 250);

            pa2 = new Panel();
            pa2.setSize(500, 250);

            Label b1 = new Label ("Select the desired end") ");
            btns = new Button[11];

            for (int a=0;a<9;a++) {
                btns[a] = new Button (a+1+"but"); // Put button name as number in button
                btns[a].addActionListener(this); // to act on each number button
                pa1.add(btns[a]); // Add buttons created in panel 1
            }

            add("South",pa1); // Create panel 1 below 
            add("North",pa2); // Create panel 2 on top
            pa2.add(lb1);

            This.addWindowListener(this); //close window
            this.setSize(500,500); // window size
            This.setVisable(true); // show window
        }
    }

public class Test1 {

    public static void main(String[] args) {
        Gugudan gu = new Gugudan ("Old Club Output").");

    }
}