Take capture of a livestream on android

I'm doing an app that receives video stream from an RTSP address and I want to take a screenshot of stream by just touching the screen ... I've already managed to make it take the screenshot but the problem is that all screenshot are black images and I really need to solve this.

btn.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            videoView.setDrawingCacheEnabled(true);

            // Este es el código importante :)
            // Sin él la vista tendrá una dimensión de 0,0 y el mapa de bits será nula
            //btn.measure(VideoView.MeasureSpec.makeMeasureSpec(0, VideoView.MeasureSpec.UNSPECIFIED),
            //        VideoView.MeasureSpec.makeMeasureSpec(0, VideoView.MeasureSpec.UNSPECIFIED));
            //btn.layout(0, 0, videoView.getMeasuredWidth(), videoView.getMeasuredHeight());
            videoView.buildDrawingCache(true);
            videoView.invalidate();

            //videoView.buildDrawingCache(true);
            final Bitmap bmp = Bitmap.createBitmap(videoView.getDrawingCache());
            String timeStamp = new SimpleDateFormat("yyyyMMdd_HHmmss").format(new Date());

            // Carpeta dónde guardamos la captura
            // En este caso, la raíz de la SD Card
            File sd = Environment.getExternalStorageDirectory();

            // El archivo que contendrá la captura
            File f = new File(sd, timeStamp+".JPEG");

            try {
                if (sd.canWrite()) {
                    f.createNewFile();
                    OutputStream os = new FileOutputStream(f);
                    bmp.compress(Bitmap.CompressFormat.JPEG, 90, os);
                    os.close();
                }
            } catch (FileNotFoundException e) {
                e.printStackTrace();
            } catch (IOException e) {
                e.printStackTrace();
            }
            videoView.setDrawingCacheEnabled(false);
            videoView.destroyDrawingCache();
        }
    });

I use a button and a videoview both the size of the screen and the button is transparent ... I wanted to do it only with the videoview but it does not react to the onclicklistener event