8-puzzle images java Frame

I get nullreferenceexception in my UI code.

idk what the problem is yesterday its works have changed some stuf and now it doesn't work anymore i think its something where i create the buttons

can someone look where i have maked the mistake

import java.awt.BorderLayout; import java.awt.Button; import java.awt.Color; import java.awt.Container; import java.awt.Dimension; import java.awt.Frame; import java.awt.Graphics; import java.awt.GridLayout; import java.awt.Image; import java.awt.List; import java.awt.MediaTracker; import java.awt.event.ActionEvent; import java.awt.event.ActionListener; import java.awt.image.BufferedImage; import java.awt.image.CropImageFilter; import java.awt.image.FilteredImageSource; import java.io.File; import java.io.IOException; import java.net.MalformedURLException; import java.net.URL; import java.util.ArrayList; import java.util.Arrays; import java.util.Comparator; import java.util.HashSet; import java.util.LinkedList; import java.util.PriorityQueue; import java.util.Random; import static javafx.scene.input.KeyCode.T; import javax.imageio.ImageIO; import javax.swing.*; public class Puzzle extends JFrame implements ActionListener { /** JPanel variable */ private JPanel centerPanel = new JPanel(); private JPanel northPanel = new JPanel(); private JPanel southPanel = new JPanel(); /** JButton variable */ private JButton newGame = new JButton("New Game"); private JButton[] btArray = new JButton[15]; List Collections = new List(15); /** JLabel variable */ private JLabel moves = new JLabel("Moves : "); private JLabel label; private Container mainPanel = this.getContentPane(); private int[][] pos; private int count; private String str; private Image source, image; int width, height; public static <T> void shuffle(T[] array) { Random random = new Random(); for (int i = array.length - 1 ; i >= 1; --i) { int j = random.nextInt(i); T temp = array[i]; array[i] = array[j]; array[j] = temp; } } public Puzzle() { super("Puzzle Game"); setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); initUI(); } public void initUI() { /* pos = new int[][]{ {0, 1, 2}, {3, 4, 5}, {6, 7, 8} */// }; pos = new int[][]{ {0, 1, 2, 3}, {4, 5, 6, 7}, {8, 9, 10, 11}, {12, 13, 14, 15}, }; ImageIcon sid = new ImageIcon(Puzzle.class.getResource("bloem.jpg")); source = sid.getImage(); width = sid.getIconWidth(); height = sid.getIconHeight(); /** Initialize button array */ for (int i = 0; i < 15; i++) { image = createImage(new FilteredImageSource(source.getSource(), new CropImageFilter(i*width/3, i*height/4, (width/3)+1, height/4))); // btArray[i] = new JButton("B" + i); btArray[i].setIcon(new ImageIcon(image)); } /** North Panel */ newGame.setFocusable(false); newGame.addActionListener(this); northPanel.setBackground(Color.red); northPanel.add(newGame); mainPanel.add(northPanel, BorderLayout.NORTH); /** South Panel */ southPanel.add(moves); southPanel.setBackground(Color.LIGHT_GRAY); mainPanel.add(southPanel, BorderLayout.SOUTH); /** Game Panel */ centerPanel.setLayout(new GridLayout(4, 4, 0, 0)); centerPanel.setBackground(Color.BLACK); /** Add actionListeners to buttons */ for (int i = 0; i < 15; i++) { btArray[i].addActionListener(this); } newGame(); mainPanel.revalidate(); label = new JLabel(""); label.setBackground(Color.BLACK); centerPanel.add(label); mainPanel.add(centerPanel); } public void newGame() { count = 0; shuffle(btArray); for (int j = 14; j >= 0; j--) { centerPanel.remove(btArray[j]); // JOptionPane.showMessageDialog(null, "pfff"); } for (int j = 14; j >= 0; j--) { centerPanel.add(btArray[j]); } centerPanel.revalidate(); } public boolean isFinished() { { if ((btArray[0].getY() == btArray[1].getY() && btArray[1].getY() == btArray[2].getY()) && (btArray[3].getY() == btArray[4].getY() && btArray[4].getY() == btArray[5].getY()) && (btArray[6].getY() == btArray[7].getY())) { if (btArray[0].getX() == btArray[3].getX() && btArray[3].getX() == btArray[6].getX() && btArray[1].getX() == btArray[4].getX() && btArray[4].getX() == btArray[7].getX() && btArray[2].getX() == btArray[5].getX()) { return true; } } return false; } } public static void main(String[] args) { Puzzle puz = new Puzzle(); puz.setBounds(20, 20, 300, 325); puz.setVisible(true); } public void actionPerformed(ActionEvent ae) { JButton button = (JButton) ae.getSource(); Dimension size = button.getSize(); if (isFinished()) { JOptionPane.showMessageDialog(null, "You have Won the game"); } if (ae.getSource() == newGame) { newGame(); } int labelX = label.getX(); int labelY = label.getY(); int buttonX = button.getX(); int buttonY = button.getY(); int buttonPosX = buttonX / size.width; int buttonPosY = buttonY / size.height; int buttonIndex = pos[buttonPosY][buttonPosX]; if (labelX == buttonX && (labelY - buttonY) == size.height) { int labelIndex = buttonIndex + 4; centerPanel.remove(buttonIndex); centerPanel.add(label, buttonIndex); centerPanel.add(button, labelIndex); centerPanel.validate(); count++; } if (labelX == buttonX && (labelY - buttonY) == -size.height) { int labelIndex = buttonIndex - 4; centerPanel.remove(labelIndex); centerPanel.add(button, labelIndex); centerPanel.add(label, buttonIndex); centerPanel.validate(); count++; } if (labelY == buttonY && (labelX - buttonX) == size.width) { int labelIndex = buttonIndex + 1; centerPanel.remove(buttonIndex); centerPanel.add(label, buttonIndex); centerPanel.add(button, labelIndex); centerPanel.validate(); count++; } if (labelY == buttonY && (labelX - buttonX) == -size.width) { int labelIndex = buttonIndex - 1; centerPanel.remove(buttonIndex); centerPanel.add(label, labelIndex); centerPanel.add(button, labelIndex); centerPanel.validate(); count++; } str = String.valueOf(count); moves.setText("Moves : " + str); } }