C# code does not detect webcam always

Question

I use fallowing library to detect webcam for simple c# application. When it debug works fine and when I try again It give fallowing error.

Used reference If somebody has any idea about it please help me to solve it.

First get this box and when click OK get fallowing error message.

an error occurred while capturing the video image. the video capture will now be terminated. Object reference not set to an instance of an object.

Form1.cs

using System;
using System.Linq;
using System.Text;
using System.Data;
using System.Drawing;
using System.Windows.Forms;
using System.ComponentModel;
using System.Collections.Generic;
using WinFormCharpWebCam;

namespace camTest
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }

        WebCam webcam;

        private void button1_Click(object sender, EventArgs e)
        {
            webcam.Start();
        }

        private void button2_Click(object sender, EventArgs e)
        {
            webcam.Stop();
        }

        private void button3_Click(object sender, EventArgs e)
        {
            webcam.Continue();
        }

        private void Form1_Load(object sender, EventArgs e)
        {
            webcam = new WebCam();
            webcam.InitializeWebCam(ref imgVideo);
        }
    }
}

Webcam.cs

using System;
using System.IO;
using System.Linq;
using System.Text;
using WebCam_Capture;
using System.Collections.Generic;



namespace WinFormCharpWebCam
{
    //Design by Pongsakorn Poosankam
    class WebCam
    {
        private WebCamCapture webcam;
        private System.Windows.Forms.PictureBox _FrameImage;
        private int FrameNumber = 30;
        public void InitializeWebCam(ref System.Windows.Forms.PictureBox ImageControl)
        {
            webcam = new WebCamCapture();
            webcam.FrameNumber = ((ulong)(0ul));
            webcam.TimeToCapture_milliseconds = FrameNumber;
            webcam.ImageCaptured += new WebCamCapture.WebCamEventHandler(webcam_ImageCaptured);
            _FrameImage = ImageControl;
        }

        void webcam_ImageCaptured(object source, WebcamEventArgs e)
        {
            _FrameImage.Image = e.WebCamImage;
        }

        public void Start()
        {
            webcam.TimeToCapture_milliseconds = FrameNumber;
            webcam.Start(0);
        }

        public void Stop()
        {
            webcam.Stop();
        }

        public void Continue()
        {
            // change the capture time frame
            webcam.TimeToCapture_milliseconds = FrameNumber;

            // resume the video capture from the stop
            webcam.Start(this.webcam.FrameNumber);
        }

        public void ResolutionSetting()
        {
            webcam.Config();
        }

        public void AdvanceSetting()
        {
            webcam.Config2();
        }

    }
}