Ошибка тестирования диалогового окна

@RunWith(AndroidJUnit4.class)
@LargeTest
public class CategoryEditorTest {
    String space = " ";
    @Rule
    public ActivityTestRule<ActivityCreateCategory> mActivityRule = new 
                            ActivityTestRule<>(ActivityCreateCategory.class);
    @Test
    public void validation_OnlySpace() throws Exception {
        //добавить пробелы в текстовое поле
        onView(withId(R.id.et_CategoryName)).perform(typeText(space), closeSoftKeyboard());
        //создать категорию
        onView(withId(R.id.btn_menu_add)).perform(click());
        ActivityCreateCategory activityCreateCategory = mActivityRule.getActivity(); //строка №73
        assertNull(activityCreateCategory);
        AlertDialog dialog = activityCreateCategory.getLastDialog();
        assertNull(dialog);
    }
}

Пишет вот такую ошибку

java.lang.AssertionError: expected null, but was:<com.astend.myshop.activitys.ActivityCreateCategory@b668b820>
at org.junit.Assert.fail(Assert.java:88)
at org.junit.Assert.failNotNull(Assert.java:755)
at org.junit.Assert.assertNull(Assert.java:737)
at org.junit.Assert.assertNull(Assert.java:747)
at com.astend.myshop.CategoryEditorTest.validation_OnlySpace(CategoryEditorTest.java:73)
...

Что я делаю не так?