The image's event handler shows error in Java GUI

I've created the r Jbutton and inserted an image into the c JButton.All I want to do is add an event handler to the r JButton that when it is clicked the image in c button to be changed as specified in the method roll.But it shows an error inside thehandler class in the line:r.roll .Can someone show me how to add the handler into the r JButton in a way that when the r JButton is clicked the method roll executes and the image is changed? This is my code:

import java.awt.*;
import java.awt.event.*;
import javax.swing.event.*;
import javax.swing.*;
import java.util.Random;
import javax.imageio.*;

public class Background extends JFrame{      
 private Random ran;
 private int value;
 private JButton r;
 private JButton c;
 
 
  public Background ()
{
  super("title");
  ran = new Random();
  value = nextValue() ;
  setLayout(new FlowLayout());
   
   r=new JButton("ROLL ");
   r.setForeground(Color.WHITE);//ndryshon ngjyren e shkrimit
   r.setBackground(Color.YELLOW);
   add(r,BorderLayout.SOUTH);
   
  
   Icon i=new ImageIcon(getClass().getResource("1.png"));
   Icon im=new ImageIcon(getClass().getResource("2.png"));
   
   
   c= new JButton(i);
    add(c);
   
    
   thehandler hand=new thehandler(this);//konstruktori i handler merr nje instance te Background
   r.addActionListener(hand);
c.addActionListener(hand);

  
    }
  
  
  private int nextValue() {
   return Math.abs(ran.nextInt()) % 6 + 1 ;
   }
  public void roll() {
   value = nextValue() ;
if (value==1){
 Icon i=new ImageIcon(getClass().getResource("1.png"));
 c= new JButton(i);
   add(c);
}
else if(value==2){
 Icon im=new ImageIcon(getClass().getResource("2.png"));
 c= new JButton(im);
   add(c);
}
  
   
   repaint() ;
   }
  public int getValue() {
   return value ;
   }
  
  private class thehandler implements ActionListener{
   private Background m ;
   thehandler(Background thisone) {
    m = thisone ; }
  
   public void actionPerformed(ActionEvent event) {
    m.roll();
   r.roll();//ERROR  
   }
   }
  public static void main(String[] args) {
   
   Background  d = new Background() ;
   d.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
   d.getContentPane().setBackground(Color.GREEN);
   d.setSize(700,500);
   d.setVisible(true);  
  }
  }