Avoid calling getCameraToPreviewMatrix() for getFacesDetected() if no faces are detected.

     *  is enabled). The face.temp rect will store the face rectangle in screen coordinates.

     */

    public CameraController.Face [] getFacesDetected() {

        if( faces_detected != null ) {

        if( faces_detected != null && faces_detected.length > 0 ) {

            // note, we don't store the screen coordinates, as they may become out of date in the

            // screen orientation changes (if MainActivity.lock_to_landscape==false)

            final Matrix matrix = getCameraToPreviewMatrix();