为什么谷歌端点方法失败？

Question

当我的端点api方法的指示行发生NullPointerException时，由android客户端调用，而不是从api资源管理器调用时：

@ApiMethod(name = "publishReview", path = "publish-review", httpMethod = ApiMethod.HttpMethod.POST)
public Review publishReview(@Named("userId") final String id, ReviewForm reviewForm) {
    Key<Profile> profileKey = Key.create(Profile.class, id);

    final Key<Review> reviewKey = factory().allocateId(profileKey, Review.class);

    final Long reviewId = reviewKey.getId();

    Profile user = ofy().load().key(profileKey).now();

    Review review = new Review(reviewId, id, reviewForm);
    user.addToMyReviews(reviewId); // NULLPOINTER HERE
    ofy().save().entities(review, user).now();

    return review;
}

以下是addToMyReviews(Long reviewId)：

public void addToMyReviews(final Long reviewId) {
    if (!myReviews.contains(reviewId))
        myReviews.add(reviewId);
}

这是端点方法的android客户端调用：

public static class PublishReview extends AsyncTask<Void, Void, String> {

    private static MyApi myApiService = null;
    private ReviewForm mReview;
    private final String mUserId;
    private Context mContext;

    public PublishReview(final String userId, ReviewForm review, Context context) {
        mReview = review;
        mUserId = userId;
        mContext = context;
    }

    @Override
    protected String doInBackground(Void... params) {
        if (myApiService == null) {  // Only do this once
            MyApi.Builder builder = new MyApi.Builder(AndroidHttp.newCompatibleTransport(),
                    new AndroidJsonFactory(), null)
                    // options for running against local devappserver
                    // - 10.0.2.2 is localhost's IP address in Android emulator
                    // - turn off compression when running against local devappserver
                    .setRootUrl("http://10.0.2.2:8080/_ah/api/")
                    .setGoogleClientRequestInitializer(new GoogleClientRequestInitializer() {
                        @Override
                        public void initialize(AbstractGoogleClientRequest<?> abstractGoogleClientRequest) throws IOException {
                            abstractGoogleClientRequest.setDisableGZipContent(true);
                        }
                    });
            myApiService = builder.build();
        }

        try {
            return myApiService.publishReview(mUserId, mReview).execute().getTitle();
        } catch (IOException e) {
            e.printStackTrace();
            return null;
        }
    }

    @Override
    protected void onPostExecute(String title) {
        Toast.makeText(mContext, title + " published", Toast.LENGTH_LONG).show();
    }
}

当作为params传递给端点方法时，客户端上的mUserId和mReview变量不为null。

如何解决此错误？