получение исключения нулевого указателя для тестового примера базы данных Android

Я хочу написать тестовый пример для модуля базы данных Android. Ниже приведен код, который я пишу.

@RunWith(AndroidJUnit4.class)
    @SmallTest
    public class DatabaseTest {
        private SQLiteDatabase mDatabase;
        private HeaterDataSource mDataSource;
        private DatabaseHelper mDbHelper;

    public ActivityTestRule<MainActivity> mActivityRule = new ActivityTestRule<>(
            MainActivity.class);

    @Before
    public void createDatabase() {
        mDbHelper = new DatabaseHelper(mActivityRule.getActivity());
        mDatabase = mDbHelper.getWritableDatabase();
        mDataSource = new HeaterDataSource(mDatabase);
    }

    //According to Zainodis annotation only for legacy and not valid with gradle>1.1:
    @Test
    public void testAddEntry(){
        Heater heater = new Heater();
        heater.setName("heater 1");
        heater.setDateTime("21 march 2016");
        heater.setState("ON");
        heater.setPeakValue("40");
        heater.setTemperature("30 degrees");
        heater.setConsumption("20 KWh");
        mDataSource.insert(heater);


        List<Heater> expenseTypes = mDataSource.read();
        assertThat(expenseTypes.size(), is(1));
        assertTrue(expenseTypes.get(0).getName().equals("heater 1"));
    }
}

Но я получаю исключение nullpointer

Запуск тестов Тестовый запуск запущен

java.lang.NullPointerException: Attempt to invoke virtual method 'android.database.sqlite.SQLiteDatabase android.content.Context.openOrCreateDatabase(java.lang.String, int, android.database.sqlite.SQLiteDatabase$CursorFactory, android.database.DatabaseErrorHandler)' on a null object reference
at android.database.sqlite.SQLiteOpenHelper.getDatabaseLocked(SQLiteOpenHelper.java:223)
at android.database.sqlite.SQLiteOpenHelper.getWritableDatabase(SQLiteOpenHelper.java:163)
at com.xyz.android.DatabaseTest.createDatabase(DatabaseTest.java:39)
at java.lang.reflect.Method.invoke(Native Method)
at org.junit.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMethod.java:50)
at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:12)
at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:47)
at org.junit.internal.runners.statements.RunBefores.evaluate(RunBefores.java:24)
at org.junit.runners.ParentRunner.runLeaf(ParentRunner.java:325)
at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:78)
at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:57)
at org.junit.runners.ParentRunner$3.run(ParentRunner.java:290)
at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:71)
at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:288)
at org.junit.runners.ParentRunner.access$000(ParentRunner.java:58)
at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:268)
at org.junit.runners.ParentRunner.run(ParentRunner.java:363)
at org.junit.runners.Suite.runChild(Suite.java:128)
at org.junit.runners.Suite.runChild(Suite.java:27)
at org.junit.runners.ParentRunner$3.run(ParentRunner.java:290)
at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:71)
at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:288)
at org.junit.runners.ParentRunner.access$000(ParentRunner.java:58)
at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:268)
at org.junit.runners.ParentRunner.run(ParentRunner.java:363)
at org.junit.runner.JUnitCore.run(JUnitCore.java:137)
at org.junit.runner.JUnitCore.run(JUnitCore.java:115)
at android.support.test.internal.runner.TestExecutor.execute(TestExecutor.java:54)
at android.support.test.runner.AndroidJUnitRunner.onStart(AndroidJUnitRunner.java:240)
at android.app.Instrumentation$InstrumentationThread.run(Instrumentation.java:1970)

Пожалуйста, помогите.