经过多次旋转后，旋转图像会导致java.lang.NullPointerException

Question

在我的应用中，我拥有它，以便button每按一次bitmap旋转90度。我有另一个button执行相同的操作，但旋转了bitmap -90度。

我的问题是，在我按下旋转buttons约4-6次后，每张图片都会有所不同，无论如何，我都会收到错误：

    `java.lang.NullPointerException`: Attempt to invoke virtual method 'int android.graphics.Bitmap.getWidth()' on a null object reference.  

为什么在前几次旋转fxn正常工作时会发生此错误？我注意到，每次按下button旋转时，位图的宽度和高度都会增加，但如果应用程序大于imageview，则该应用程序不应该崩溃。我尝试了多种其他方法，但它们似乎崩溃或只是缩小图像。

这是我的代码：

    rotateClockWise.setOnClickListener(new View.OnClickListener(){
        @Override
        public void onClick(View v) {
            Bitmap bitmap = imageViewtoBitmap(imageView);
            int width = bitmap.getWidth();
            Log.i(TAG, Integer.toString(width));
            int height = bitmap.getHeight();
            Log.i(TAG, Integer.toString(height));
            Matrix matrix = new Matrix();

            matrix.postRotate(90);
            Bitmap rotatedBitmap = Bitmap.createBitmap(bitmap, 0, 0,
                    width, height, matrix, true );

            imageView.setImageBitmap(rotatedBitmap);

        }
    });

    rotateCCW.setOnClickListener(new View.OnClickListener(){
        @Override
        public void onClick(View v) {
            Bitmap bitmap = imageViewtoBitmap(imageView);
            int width = bitmap.getWidth();
            Log.i(TAG, Integer.toString(width));
            int height = bitmap.getHeight();
            Log.i(TAG, Integer.toString(height));
            Matrix matrix = new Matrix();
            matrix.postRotate(-90);

            Bitmap rotatedBitmap = Bitmap.createBitmap(bitmap, 0, 0,
                    bitmap.getWidth(), bitmap.getHeight(), matrix, true );

            imageView.setImageBitmap(rotatedBitmap);

        }
    });

public static Bitmap imageViewtoBitmap(ImageView imageView){
    /**
     * Args:
     *     imageview: an imageview
     * Returns:
     *     the bitmap of the imageview
     */

    //convert imageview to bitmap
    imageView.setDrawingCacheEnabled(true);

    // Without it the view will have a dimension of 0,0 and the bitmap will 
 be null
    imageView.measure(View.MeasureSpec.makeMeasureSpec(0, 
    View.MeasureSpec.UNSPECIFIED),
            View.MeasureSpec.makeMeasureSpec(0, 
    View.MeasureSpec.UNSPECIFIED));
    // hardcoded so i always know how big image is
    imageView.layout(0, 0, imageView.getMeasuredWidth(), 
    imageView.getMeasuredHeight());
    if (imageView == null){
        Log.i(TAG, "nothing");
    }
    imageView.buildDrawingCache(true);
    Bitmap bitmapImage = Bitmap.createBitmap(imageView.getDrawingCache());
    imageView.setDrawingCacheEnabled(false); // clear drawing cache

    return bitmapImage;


}