C# Image Capture Using Webcam , Object Reference Not Set

Question

I want to Capture Image Using Webcam. I am using WebCam_Capture.dlllibrary.

sometimes following code work Perfect. (Mostly after restarting system or turn on and off Vcamp.exe)

sometimes Camera won't turn on and It seems an Image captured of Green color

sometimes camera won't turn on and a message-box Shows with description "Object reference not set to an instance of an object" message always on webcam.Start(0); in capture()

I want to know the cause of error. How to solve this problem

---

global Variables

private WebCamCapture webcam;
private System.Windows.Forms.PictureBox _FrameImage;
private int FrameNumber = 50;
bool FCapture=true;

Webcam Inetilization

 public void InitializeWebCam(ref System.Windows.Forms.PictureBox ImageControl)
    {
        try
        {
            webcam.Dispose();
        }
        catch (Exception)
        { 
        }
        webcam = new WebCamCapture();
        webcam.FrameNumber = ((ulong)(0ul));
        webcam.TimeToCapture_milliseconds = FrameNumber;
        webcam.ImageCaptured += new WebCamCapture.WebCamEventHandler(webcam_ImageCaptured);
        _FrameImage = ImageControl;
    }

Webcam Capturing Function

    void webcam_ImageCaptured(object source, WebcamEventArgs e)
    {
        if (FCapture == true)
        {
            imgbox1.Image = e.WebCamImage;
            FCapture = false;
        }
        else
        {
            webcam.Stop();
        }
    }

Call function.

public void capture()
    {

        InitializeWebCam(ref _FrameImage);
        webcam.TimeToCapture_milliseconds = FrameNumber;
        webcam.Start(0);

    }