关于C#实现本机摄像头拍照的执行报错

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private void button1_Click(object sender, EventArgs e)
        {
            if (videoSource == null)
                return;
            Bitmap bitmap = videoSourcePlayer1.GetCurrentVideoFrame();
            string fileName = "wqf.jpg";//DateTime.Now.ToString("yyyy-MM-dd-HH-mm-ss-ff") + ".jpg";
            bitmap.Save(@"d:\temp\" + fileName,ImageFormat.Jpeg);//此处报错,未将对象引用设置到对象的实例,求助各位高手
            bitmap.Dispose();
        }