NullPointerException in update(Graphics g)

Why am I always getting NullPointerExceptions in this update method:


public synchronized void update(Graphics g) {

    Color plotColor;

    left = 10;
    right = monitor.location().x-5;
    floor = bottomPanel.location().y-50;
    length = right - left;
    inc = length/11.00;

    if(offscreenG == null) {
        offscreenImg = createImage(size().width, size().height);
        offscreenG = offscreenImg.getGraphics();
    }

    if(trajG == null) {
        trajImg = createImage(size().width, size().height);
        trajG = trajImg.getGraphics();
        drawOldTrajectories(trajG);
    }

    offscreenG.drawImage(trajImg,0,0,this);

    if(running) drawCurrentTrajectory(offscreenG);
   
    drawFloor(offscreenG);
    drawCannon(offscreenG);
    g.drawImage(offscreenImg,0,0,this);
}

  All of the drawXxx methods are declared synchronized.  I get the exception within these draw methods, usually the last time my run() method calls it (the update).  
Here is my run() method:

public void run() {

    Double tempDouble;
    tempDouble = new Double(angleField.getText().trim());
    theta = (tempDouble.doubleValue())*Math.PI/180;
    double c1 = Math.tan(theta);
    int vel = this.vel.getValue();
    double c2 = g/(2*vel*vel*Math.cos(theta)*Math.cos(theta));
    plotX = plotY = 0;
    points.removeAllElements();

    double R = c1/c2;


    long t = System.currentTimeMillis();
    for(double i = 0; i<=R/2; i+=(5/inc) ){
        plotX = (int)(inc*i) + left;
        plotY = floor - (int)(inc*(c1*i - c2*i*i));
        points.addElement(new Point(plotX, plotY));
        repaint();
        try {
            t+=sleepTime;
            Thread.sleep(Math.max(0, t - System.currentTimeMillis()));
            }
        catch (InterruptedException e) {}
    }

    peak = (vel*vel*Math.sin(theta)*Math.sin(theta))/(2*g);
    heightField.setText(""+peak);
    repaint();

    for(double i = R/2; i<=R; i+=(5/inc) ){
        plotX = (int)(inc*i) + left;
        plotY = floor - (int)(inc*(c1*i - c2*i*i));
        points.addElement(new Point(plotX, plotY));
        repaint();
        try {
            t+=sleepTime;
            Thread.sleep(Math.max(0, t - System.currentTimeMillis()));
            }
        catch (InterruptedException e) {}
    }



    plotX = left + (int)(inc*R);
    plotY = floor - (int)(inc*(c1*R - c2*R*R));
    monitor.property[trial][2].setText(""+R);
    repaint();
   
    allPoints.setElementAt((Vector)points.clone(),trial);
    running = false;
    offscreenG = null;
    trajG = null;
    repaint();

}