why black resulted image while cropping largest contour from image ?

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@Override
public Mat onCameraFrame(Mat inputFrame) {

    source =inputFrame;
    contours = new ArrayList<MatOfPoint>();
    hierarchy = new Mat();
    Imgproc.cvtColor(source, mGray, Imgproc.COLOR_RGB2GRAY);
    Imgproc.GaussianBlur(mGray, mGray1, new Size(9, 9), 1);
    segImage=imageSegmentation(mGray1);
    Mat kernel = Imgproc.getStructuringElement(Imgproc.MORPH_RECT,new Size(11,11));
    Imgproc.dilate(segImage, mGray2, kernel);
    Imgproc.findContours(mGray2, contours, hierarchy, Imgproc.RETR_TREE, Imgproc.CHAIN_APPROX_SIMPLE, new Point(0, 0));
    hierarchy.release();
    //Imgproc.drawContours(source, contours, -1, new Scalar(Math.random()*255, Math.random()*255, Math.random()*255));
    double maxVal = 0;
    maxValIdx = 0;
    for (int contourIdx = 0; contourIdx < contours.size(); contourIdx++)
    {
        double contourArea = Imgproc.contourArea(contours.get(contourIdx));
        if (maxVal < contourArea)
        {
            maxVal = contourArea;
            maxValIdx = contourIdx;
        }
    }
    Imgproc.drawContours(source, contours, maxValIdx, new Scalar(0,255,0), 5);
    //extract the maximum region
    Rect rect = Imgproc.boundingRect(contours.get(maxValIdx));
    cropped = source.submat(rect);
    Log.i("dd",cropped+"");
    return cropped;
}