I have drawn and saved the Rectangle on the image i loaded in the picture box. How i like to draw multiple rectangles for that i tried array in the rectangle but it gives error ("Object reference not set to an instance of an object." (Null reference Exception was unhandled).

enter code here
private void pictureBox1_MouseDown(object sender, MouseEventArgs e)
    {
        if (mybitmap == null)
        {
            mybitmap = new Bitmap(sz.Width, sz.Height);

        }
        rect = new Rectangle(e.X, e.Y, 0, 0);
        this.Invalidate();


    }

private void pictureBox1_MouseMove(object sender, MouseEventArgs e) {

       if (stayToolStripMenuItem.Checked == true)
       {
           switch (e.Button)
           {
               case MouseButtons.Left:
                   {
                       rect = new Rectangle(rect.Left, rect.Top, e.X - rect.Left, e.Y - rect.Top);
                       pictureBox1.Invalidate();
                        break;
                   }


           }
       }

    }

    private void pictureBox1_Paint(object sender, PaintEventArgs e)
    {
        if (stayToolStripMenuItem.Checked == true)
        {
            button1.Visible = true;
            button2.Visible = true;

            if (mybitmap == null)
            {
                return;
            }


            using (g = Graphics.FromImage(mybitmap))
            {

                using (Pen pen = new Pen(Color.Red, 2))
                {
                    //g.Clear(Color.Transparent);


                    e.Graphics.DrawRectangle(pen, rect);
                    label1.Top = rect.Top; label1.Left = rect.Left; label1.Width = rect.Width;
                    label1.Height = rect.Height;


                        if (label1.TextAlign == ContentAlignment.TopLeft)
                        {
                            e.Graphics.DrawString(label1.Text, label1.Font, new SolidBrush(label1.ForeColor), rect);
                            g.DrawString(label1.Text, label1.Font, new SolidBrush(label1.ForeColor), rect);
                            g.DrawRectangle(pen, rect);

                        }

} } } }

How can i do this.....