从ImageView里取出图片之后后变换大小报错

 ImageView oldImageView = (ImageView) currentView.findViewById(R.id.imageview);            oldImageView.setDrawingCacheEnabled(true);            Bitmap oldBitmap = oldImageView.getDrawingCache();            oldImageView.setDrawingCacheEnabled(false);            Bitmap newBitmap = exchangeBitmapSize(oldBitmap, 0.75f, 0.75f);....//变换图片的方法public static Bitmap exchangeBitmapSize(Bitmap bitmap, float scaleX, float scaleY) {        int width = bitmap.getWidth();        int height = bitmap.getHeight();        Matrix matrix = new Matrix();        matrix.postScale(scaleX, scaleY);        //下面这行代码报错        return Bitmap.createBitmap(bitmap, 0, 0, width, height, matrix, true);    }