Why this null pointer exception when using BufferStrategy with swing timer in Java?

Why might I be getting the following exception with the below code?

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException

at java.awt.Component$BltBufferStrategy.showSubRegion(Unknown Source)

at java.awt.Component$BltSubRegionBufferStrategy.show(Unknown Source)

at javax.swing.BufferStrategyPaintManager.flushAccumulatedRegion(Unknown Source)

...

It only happens about every other time I run it but always right at the start. I'm using if(bs.contentLost()){...} so I don't understand why it would be having problems.

import java.awt.*;

import java.awt.event.*;

import java.awt.image.*;

import javax.swing.*;

import java.util.Random;



public class MLM2 extends JFrame implements ActionListener {

    private final Timer timer = new Timer(20, this);

    private Insets insets;

    private BufferStrategy bs;

    private BufferedImage drawing;

    int lastW;

    int lastH;

    int pos = 0;



    public static void main (String[] args) {

        MLM2 ex = new MLM2();

    }



    public void actionPerformed(ActionEvent e) {

        if(bs.contentsLost()) return;

        Graphics2D g = null;

        g = (Graphics2D)bs.getDrawGraphics();

            int w = getWidth() - insets.left - insets.right;

            int h = getHeight() - insets.top - insets.bottom;

            if(w!=lastW || h!=lastH) {

                drawing = (BufferedImage) this.createImage(w,h);

                lastW = w;

                lastH = h;

            }

            Graphics2D drawingBoard = drawing.createGraphics();

            drawingBoard.setColor(Color.PINK);

            drawingBoard.setRenderingHint(RenderingHints.KEY_TEXT_ANTIALIASING, RenderingHints.VALUE_TEXT_ANTIALIAS_ON);

            drawingBoard.setRenderingHint(RenderingHints.KEY_ANTIALIASING, RenderingHints.VALUE_ANTIALIAS_ON);

            drawingBoard.fillRect(0, 0, w, h);



            drawingBoard.setColor(Color.RED);

            drawingBoard.fillRect(pos, 100, 100, 100);



            pos++;

            if(pos>=w) pos=0;



            g.drawImage(drawing, insets.left, insets.top, null);



            drawingBoard.dispose();



            if (!bs.contentsLost()) {

                bs.show();

            }

   }



    public MLM2() {

        super();



        setTitle("Mirror Land");

        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);



        setSize(800, 420);

        setLocationRelativeTo(null);

        insets = this.getInsets();

        int insetWide = insets.left + insets.right;

        int insetTall = insets.top + insets.bottom;

        setSize(getWidth() + insetWide, getHeight() + insetTall);



        setVisible(true);

        setIgnoreRepaint(true);



        createBufferStrategy(2);

        bs = getBufferStrategy();



        drawing = (BufferedImage) this.createImage(getWidth(),getHeight());



        timer.start();

    }

}