unable to destroy activity Android

here is my java code

    @Override    protected void onDestroy() {    int i;    super.onDestroy();    if (bitmapList != null) {        for (i = INDEX_COLLAGE; i < bitmapList.length; i += INDEX_COLLAGE_BACKGROUND) {            if (bitmapList[i] != null) {                bitmapList[i].recycle();            }        }    }    if (collageView != null) {        if (collageView.shapeLayoutList != null) {            for (i = INDEX_COLLAGE; i < collageView.shapeLayoutList.size(); i += INDEX_COLLAGE_BACKGROUND) {                for (int j = INDEX_COLLAGE; j < collageView.shapeLayoutList.get(i).shapeArr.length; j += INDEX_COLLAGE_BACKGROUND) {                    if (collageView.shapeLayoutList.get(i).shapeArr[j] != null) {                        collageView.shapeLayoutList.get(i).shapeArr[j].freeBitmaps();                    }                }            }        }        if (collageView.maskBitmapArray != null) {            for (i = INDEX_COLLAGE; i < collageView.maskBitmapArray.length; i += INDEX_COLLAGE_BACKGROUND) {                if (collageView.maskBitmapArray[i] != null) {                    if (!collageView.maskBitmapArray[i].isRecycled()) {                        collageView.maskBitmapArray[i].recycle();                    }                    collageView.maskBitmapArray[i] = null;                }            }        }    }    if (adWhirlLayout != null) {        adWhirlLayout.removeAllViews();        adWhirlLayout.destroy();    }}private void backButtonAlertBuilder() {    AlertDialog.Builder builder = new AlertDialog.Builder(CreateCollageActivity.this);    builder.setMessage("Would you like to save image ?").setCancelable(true).setPositiveButton("Yes", new OnClickListener() {        public void onClick(DialogInterface dialog, int id) {            if (analytics != null)                analytics.logEvent(Analytics.Param.IMAGE_SAVE, "");            new SaveImageTask().execute();        }}).setNeutralButton("No", new OnClickListener() {        public void onClick(DialogInterface dialog, int which) {            finish();        }    });

The app crashes when i click back button..when i debugged the code it showed java.lang.RuntimeException: Unable to destroy activity and HVDactivities.CreateCollageActivity.onDestroy(CreateCollageActivity.java:740)