使用位图/ ProfilePictureView到位图时的NPE

Question

我试图从facebook的profilepictureview创建一个圆形位图，但我得到一个我无法弄清楚的nullpointer：

ImageView fbimage = ( ( ImageView)userProfilePictureView.getChildAt( 0));
    Bitmap bitmap  = ( ( BitmapDrawable) fbimage.getDrawable()).getBitmap();

    Bitmap circleBitmap = Bitmap.createBitmap(bitmap.getWidth(),bitmap.getHeight(),Bitmap.Config.ARGB_8888);
    BitmapShader shader = new BitmapShader(bitmap, Shader.TileMode.CLAMP, Shader.TileMode.CLAMP);
    Paint paint = new Paint();
    paint.setShader(shader);
    paint.setAntiAlias(true);
    Canvas c = new Canvas(circleBitmap);
    c.drawCircle(bitmap.getWidth()/2,bitmap.getHeight()/2,bitmap.getWidth()/2,paint);
    fbimage.setImageBitmap(circleBitmap);

我在这条线上得到了NPE：

Bitmap bitmap  = ( ( BitmapDrawable) fbimage.getDrawable()).getBitmap();

这是我的班级（以及完整的堆栈）：https://gist.github.com/RidouaneHicham/edce609a4f5aaa59af9a

谢谢！