NullPointerException error when using Robolectric to write android tests

Question

Test code:

@RunWith(RobolectricTestRunner.class)
@Config(constants = BuildConfig.class)
public class MainActivityTest {
private MainActivity mainActivity;

@Test
public void testMainActivity() {
    MainActivity mainActivity = Robolectric.setupActivity(MainActivity.class);
    mainActivity.findViewById(R.id.visual_button).performClick();
    Intent expectedIntent = new Intent(mainActivity, VisualActivity.class);
    ShadowActivity shadowActivity = Shadows.shadowOf(mainActivity);
    Intent actualIntent = shadowActivity.getNextStartedActivity();
    Assert.assertEquals(expectedIntent, actualIntent);
}
}

error reports:

java.lang.NullPointerException

my Robolectric version is 3.1.4... l have tried to find some solutions but failed. l'm sure that my test code are under src/test, not src/androidTest, can anybody help me ? Thanks in advance!