public ReplayData replayData;

    frame = new JFrame();
    frame.setBounds(100, 100, 1920, 1080);
    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    frame.getContentPane().setLayout(null);

    // ERROR HERE WHEN ADDING TO FRAME, APPLICATION RUNS FINE IF COMMENTED
    //frame.add(replayData); // Add replay data to jframe

    JButton button_KMeans = new JButton("View K-Means");
    button_KMeans.addActionListener(new ActionListener() {
        public void actionPerformed(ActionEvent arg0) {
            kMeans.initialize();
            kMeans.kMeanCluster();
            kMeans.PrintResults();

            //for (Point p : kMeans.getPoints() )

            Point temp = new Point();
            for (int i = 0; i < kMeans.TOTAL_DATA; i++)
            {
                //JOptionPane.showMessageDialog(new JFrame(),kMeans.TOTAL_DATA, "Dialog",
                //        JOptionPane.ERROR_MESSAGE);;
                p.setX((int)TrackerData.getRecordNumber(i).getEyeX());
                p.setY((int)TrackerData.getRecordNumber(i).getEyeY());

                JOptionPane.showMessageDialog(new JFrame(),p.getX(), "Dialog",
                        JOptionPane.ERROR_MESSAGE);
                JOptionPane.showMessageDialog(new JFrame(),p.getY(), "Dialog",
                        JOptionPane.ERROR_MESSAGE);

                // GET ERROR HERE when adding these points to replayData. everything look fine in that class unless i'm missing something
                // java.lang.NullPointerException at MainWindow$3.actionPerformed(MainWindow.java:189)
                replayData.addPoint(p); // Add points to JPanel
            }
            //replayData.draw();
        }
    });

import java.awt.Graphics;
import java.awt.Point;
import java.util.ArrayList;
import javax.swing.JPanel;

public class ReplayData extends JPanel {

public ArrayList<DataPoint> points;

// Initialise records
public ReplayData()
{
    points = new ArrayList<DataPoint>();
}

public void ReplaceData() {
    points = new ArrayList<DataPoint>();
}

public void addPoint(DataPoint point) {
    points.add(point);
}

@Override
public void paintComponent(Graphics g) {
    super.paintComponent(g);

    for (DataPoint p : points)
        g.fillRect(p.x, p.y, 2, 2);
}

public void draw() {
    repaint();
}

public DataPoint(int X, int Y)
{
    x = X;
    y = Y;
}

public int getX() {
    return x;
}

public void setX(int x) {
    this.x = x;
}

public int getY() {
    return y;
}

public void setY(int y) {
    this.y = y;
}

int x,y;