Java NullPointerException (need some timer)

Question

I draw simple splines and I got code here. I editted code and receive NullPointerException in dataset.size(). I guess that my programm don't be at time filling all dataset points from files (Scanner works slowly) and it throws this error. I also guess that I need to add certain timer to wait while dataset == null. But how to make it?

public class SimpleGrapher2 extends JPanel {
    ...
    private static List<Double> scores;
    private static File[] sFiles;
    ...
    private static List<List<Point2D.Double>> dataset;
    private static int snumber = 0;

    public SimpleGrapher2(List<Double> scores) {
        SimpleGrapher2.scores = scores;
        addMouseMotionListener(new MouseMotionListener() {
            @Override
            public void mouseDragged(MouseEvent me) {
                double x = me.getX();
                double y = me.getY();
                List<Point2D.Double> series = findNearPoint(dataset, x, y);
                editSerie(x, y, series);
                revalidate();
                repaint();
            }

            @Override
            public void mouseMoved(MouseEvent me) {}
        });
    }

    static JPanel paintingComponent = new JPanel() {
        @Override
        protected void paintComponent(Graphics g) {
            super.paintComponent(g);
            Graphics2D g2 = (Graphics2D) g;
            //painting X,Y axises
            drawSplines(g2); // painting splines
        }
    };

    public static void drawSplines(Graphics2D g2) {
        int precision = 10;
        for (int k = 0; k < dataset.size(); k++) { // NULLPOINTEREXCEPTION HERE
            List<Point2D.Double> series = dataset.get(k);
            int np = series.size();
            //algorithm and painting of splines
        }
    }

    public List<Point2D.Double> findNearPoint(List<List<Point2D.Double>> dataset, double x, double y) {
        //hidden part of code: to move points
    }

    public void editSerie(double x, double y, List<Point2D.Double> serie) {
        //hidden part of code: edit series in case of MouseDragged
    }

    public void readFromFiles() {
        dataset = new ArrayList<>();
        for (File polFile : sFiles) {
            List<Point2D.Double> series = new ArrayList<>();
            Scanner s = null;
            try {
                s = new Scanner(new File(polFile.getAbsolutePath()));
            }catch (FileNotFoundException ex) {
                System.out.println("Scanner error!");
            }
            s.useLocale(Locale.US);
            while (s.hasNext()) {
                double x = s.nextDouble();
                double y = s.nextDouble();
                series.add(new Point2D.Double(x, y));
            }
            dataset.add(series);
        }
    }

    //hidden part of code: some helpfull functions

    private static void createAndShowGui() {
        ...
        SimpleGrapher2 mainPanel = new SimpleGrapher2(scores);
        mainPanel.setPreferredSize(new Dimension(800, 600));
        JFrame frame = new JFrame("DrawGraph");
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.getContentPane().add(mainPanel);
        //frame.setContentPane(new GLG2DCanvas(paintingComponent));
        frame.setContentPane(paintingComponent);
        frame.pack();
        frame.setLocationRelativeTo(null);
        frame.setVisible(true);
    }

    public static void main(String[] args) {
      SwingUtilities.invokeLater(new Runnable() {
         @Override
         public void run() {
            createAndShowGui();
         }
      });
   }
}