Android : Null pointer exception on bitmap while scrolling

I am creating a bitmap which hold Facebook image cropped in a circular shape the code was working fine but wen there is a lot of images in the same activity its giving a null pointer exception while scrolling here is my code

//enter code here
byte[] decodedString = Base64.decode(obj.picture, Base64.DEFAULT);
                            Bitmap decodedByte =  BitmapFactory.decodeByteArray(decodedString, 0, decodedString.length);
                     on this line i am getting a null pointer exception
                            Bitmap circleBitmap = Bitmap.createBitmap(decodedByte.getWidth(), decodedByte.getHeight(), Bitmap.Config.ARGB_8888);
                            BitmapShader shader = new BitmapShader(decodedByte, Shader.TileMode.CLAMP, Shader.TileMode.CLAMP);
                            Paint paint = new Paint();
                            paint.setShader(shader);
                            Canvas c = new Canvas(circleBitmap);
                            c.drawCircle(decodedByte.getWidth() / 2, decodedByte.getHeight() / 2, decodedByte.getWidth() / 2, paint);
                            holder.civilianImage.setImageBitmap(circleBitmap);

i can see the image presented but while i scroll this error is shown. Any help would be appreciated. And one more thing i getting the image from Facebook but each image is different from the other one in resolution and while cropping the image is losing a lot of it how can i manage this also.