Eu quero escrever um teste de unidade. Portanto, preciso de MutableLiveData. Comecei com um teste muito básico para instalação, mas não consigo instanciar um objeto MutableLiveData. Eu sempre sou nulo quando executo o teste. Eu tenho que zombar de alguma coisa? Alguma sugestão?

@RunWith(MockitoJUnitRunner.class)
public class DefaultLiveDataTest {

    private static final int EXPECTED = 5;

    private final MutableLiveData<Integer> underTest = new MutableLiveData<>();


    @Test
    public void exampleTest() {

    underTest.setValue(EXPECTED); //underTest is Null
    assertEquals(underTest.getValue().intValue(), EXPECTED);

    }
}

java.lang.NullPointerException
at android.arch.core.executor.DefaultTaskExecutor.isMainThread(DefaultTaskExecutor.java:58)
at android.arch.core.executor.ArchTaskExecutor.isMainThread(ArchTaskExecutor.java:116)
at android.arch.lifecycle.LiveData.assertMainThread(LiveData.java:434)
at android.arch.lifecycle.LiveData.setValue(LiveData.java:279)
at android.arch.lifecycle.MutableLiveData.setValue(MutableLiveData.java:33)
at com.mypackage.DefaultLiveDataTest.test_that_live_data_has_default_value(DefaultLiveDataTest.java:22)

build.gradle

apply plugin: 'com.android.application'

android {
compileSdkVersion 27
defaultConfig {
    applicationId 'com.mypackage.title'
    minSdkVersion 16
    targetSdkVersion 27
    versionCode 1
    versionName '1.0'
    testInstrumentationRunner 'android.support.test.runner.AndroidJUnitRunner'
}

testOptions {
    unitTests.returnDefaultValues = true
}

dependencies {
implementation fileTree(dir: 'libs', include: ['*.jar'])
implementation 'com.android.support:appcompat-v7:27.1.1'
implementation 'com.android.support:design:27.1.1'
implementation 'com.android.support.constraint:constraint-layout:1.0.2'
implementation 'com.android.support:support-vector-drawable:27.1.1'
implementation 'android.arch.lifecycle:extensions:1.1.1'
implementation 'android.arch.lifecycle:viewmodel:1.1.1'
implementation 'android.arch.lifecycle:livedata:1.1.1'

annotationProcessor 'android.arch.lifecycle:compiler:1.1.1'
testImplementation 'junit:junit:4.12'
testImplementation 'org.mockito:mockito-core:1.10.19'
androidTestImplementation 'com.android.support.test:runner:1.0.1'
androidTestImplementation 'com.android.support.test.espresso:espresso-core:3.0.1'
annotationProcessor 'org.androidannotations:androidannotations:4.4.0'
implementation 'org.androidannotations:androidannotations-api:4.4.0'
compileOnly 'org.projectlombok:lombok:1.16.20'
annotationProcessor 'org.projectlombok:lombok:1.16.20'
}