Testing Android Realm with RxJava - "opened from a thread without a Looper" Exception

I have the following code, based off the documentation provided by Realm (http://ift.tt/1SNAQlg)

public Observable<Foo> getFooById(String id) {

    realm = Realm.getInstance(realmConfiguration);

    return realm.where(Foo.class)
            .equalTo("id", id)
            .findFirstAsync()
            .asObservable()
            .filter(this::filterResult);
}

This works in App as expected however when it comes to testing things become a little more tricky. I have the following test (stripped down to keep it simple) :

@Test
public void testRealmExample() {

    RealmConfiguration config = new RealmConfiguration.Builder(context)
            .name("test.realm")
            .inMemory()
            .build();

    DataManager dataManager = new DataManager(config);

    TestSubscriber<Foo> testSubscriber = new TestSubscriber<>();
    dataManager.getFoo("").observeOn(AndroidSchedulers.mainThread()).subscribe(testSubscriber);
    testSubscriber.assertNoErrors();
}

When the test is executed the following error occurs java.lang.IllegalStateException: Your Realm is opened from a thread without a Looper. Async queries need a Handler to send results of your query.

To counter this I read on the Realm GitHub page that they use an annotation @UiThreadTest to force the test to run on the UI thread, which from my understanding is a looper thread, therefore this should solve my problem. I added:

@Rule
public final UiThreadTestRule uiThreadTestRule = new UiThreadTestRule();

and changed my test to include the annotation

@Test
@UiThreadTest
public void testRealmExample() { ...}

This still yields the same exception. Does anyone know why and a solution? Thanks.