check a button color in another button

Question

        JToggleButton start = new JToggleButton("start block");
        start.setBounds(50, 50, 100, 100);

        JToggleButton end = new JToggleButton("end block");
        end.setBounds(50, 50, 100, 100);               


        CreateSelectionButtons(frame, panelOnTheLeft, size, end,start);


    public static void CreateSelectionButtons(JFrame frame, JPanel panelOnTheLeft, int[] size, AbstractButton end,AbstractButton start) {
        int buttonNum = size[0] * size[1];
        JButton[] buttons = new JButton[buttonNum];
        class Actions implements ActionListener {
            public void actionPerformed(ActionEvent e) {
                // Get the button that was clicked
                JButton theButton = (JButton) e.getSource();

                // Set it's background color to white
                Color color = theButton.getBackground();
                if(start.isSelected()){
                    for(int i=0;i<=buttonNum;i++){
                        Color check=buttons[i].getBackground();
                        if(check==Color.green){
                            buttons[i].setBackground(Color.white);

                        }
                    }
                    theButton.setBackground(Color.green);}
                else if(end.isSelected()){
                    for(int i=0;i<=buttonNum;i++){
                        Color check=buttons[i].getBackground();
                        if(check==Color.red){
                            buttons[i].setBackground(Color.white);

                        }
                    }
                    theButton.setBackground(Color.red);}
                else if (color==Color.black)
                {
                    theButton.setBackground(Color.white);}
                else{theButton.setBackground(Color.black);
                }

            }}


        public static void main(String[] args) {
            JFrame frame = new JFrame("Maze Program");
            JPanel panelOnTheLeft = new JPanel();
            JPanel panelOnTheRight = new JPanel();

            frame.setLayout(new BorderLayout());
            CreateSelectionButtons(frame, panelOnTheLeft, defaultSize, null,null);
            CreateInputSection(frame, panelOnTheLeft, panelOnTheRight);
            frame.add(panelOnTheLeft, BorderLayout.WEST);
            frame.add(panelOnTheRight, BorderLayout.EAST);

            frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
            frame.pack();
            frame.setVisible(true);

        }
    }

i want the button on the left will change color depent on the the button i clicked (start and end),but i get

exception in thread "AWT-EventQueue-0" java.lang.NullPointerException: Cannot invoke "javax.swing.AbstractButton.isSelected()" because "this.val$start" is null)

what should i do