Null reference exception

Question

below is my code: i am getting an error null reference exception was unhandled on following highlighted line

using System;

using System.Collections.Generic;

using System.ComponentModel;

using System.Data;

using System.Drawing;

using System.Linq;

using System.Text;

using System.Windows.Forms;

using Emgu.CV;

using Emgu.CV.Structure;

using Emgu.Util;

using Emgu.CV.CvEnum;

namespace CameraCapture

{

    public partial class CameraCapture : Form

    {

        //declaring global variables

        private Capture capture;        //takes images from camera as image frames

        private HaarCascade haar;            //the viola-jones classifier (detector)

        public CameraCapture()

        {

            InitializeComponent();

        }

        //------------------------------------------------------------------------------//

        //Process Frame() below is our user defined function in which we will create an EmguCv 

        //type image called ImageFrame. capture a frame from camera and allocate it to our 

        //ImageFrame. then show this image in ourEmguCV imageBox

        //------------------------------------------------------------------------------//

        private void ProcessFrame(object sender, EventArgs arg)

        {

            //Read an image from hard disk at location e.g 'E:\IMAGES\'

            //Image InputImg = Image.FromFile(@"E:\IMAGES\MyPic.jpg");

            //Image<Bgr, byte> MyImage = new Image<Bgr, byte>(new Bitmap(InputImg));

            //fetch the frame captured by web camera

            Image<Bgr, Byte> ImageFrame = capture.QueryFrame();

            //Detect Faces from the ImageFrame

            if (ImageFrame != null)   // confirm that image is valid

            {

                //convert the image to gray scale

                Image<Gray, byte> grayframe = ImageFrame.Convert<Gray, byte>();

            

                //detect faces from the gray-scale image and store into an array of type 'var',i.e 'MCvAvgComp[]'

      

                

                    var faces = grayframe.DetectHaarCascade(haar, 1.4, 4,

                                            HAAR_DETECTION_TYPE.DO_CANNY_PRUNING,

                                            new Size(25, 25))[0];

         

                //draw a green rectangle on each detected face in image

                foreach (var face in faces)

                {

                    ImageFrame.Draw(face.rect, new Bgr(Color.Green), 3);

                }

            }

            //show the image in the EmguCV ImageBox

            CamImageBox.Image = ImageFrame;

            //To show the image in windows forms PictureBox called pictureBox1

            //pictureBox1.Image= ImageFrame.ToBitmap(); 

        }

        //btnStart_Click() function is the one that handles our "Start!" button's click 

        //event. it creates a new capture object if its not created already. e.g at first time

        //starting. once the capture is created, it checks if the capture is still in progress,

        private void btnStart_Click(object sender, EventArgs e)

        {

            if (capture != null)

            {

                if (btnStart.Text == "Pause")

                {  //if camera is getting frames then pause the capture and set button Text to

                    // "Resume" for resuming capture

                    btnStart.Text = "Resume"; //

                    Application.Idle -= ProcessFrame;

                }

                else

                {

                    //if camera is NOT getting frames then start the capture and set button

                    // Text to "Pause" for pausing capture

                    btnStart.Text = "Pause";

                    Application.Idle += ProcessFrame;

                }

            }

        }

        private void ReleaseData()

        {

            if (capture != null)

                capture.Dispose();

        }

        /// <summary>

        /// This function is the Selected Index changed event of the comboBox

        /// It allows the user to select a desired camera and starts it for him/her

        /// </summary>

        private void cbCamIndex_SelectedIndexChanged(object sender, EventArgs e)

        {

            //Set the camera number to the one selected via combo box

            int CamNumber = -1;

            CamNumber = int.Parse(cbCamIndex.Text);

            //Start the selected camera

            #region if capture is not created, create it now

            if (capture == null)

            {

                try

                {

                    capture = new Capture(CamNumber);

                }

                catch (NullReferenceException excpt)

                {

                    MessageBox.Show(excpt.Message);

                }

            }

            #endregion

            //Start showing the stream from camera

            btnStart_Click(sender, e);

            btnStart.Enabled = true;

        }

        private void CameraCapture_Load(object sender, EventArgs e)

        {

            // adjust path to find your xml at loading

            haar = new HaarCascade("haarcascade_frontalface_alt_tree.xml");

        }

    }

}