Android testing - NoClassDefFoundError of my Activity subclassing FragmentActivity

I'm developing an Android application using Android Studio, multidex feature and with the support library to use FragmentActivityand Fragment.

I've set up unit tests with the following Gradle configuration:

android {
    // ...
    testInstrumentationRunner "android.support.test.runner.AndroidJUnitRunner"
}
// ...
dependencies {
    // ...
    androidTestCompile('com.android.support.test:runner:0.3')
    // ...
}

Considering the following Activity:

public class MainActivity extends android.support.v4.app.FragmentActivity {

When I wrote a test class to test it with Robotium, I declared it like:

public class TestRobotiumMainActivity extends ActivityInstrumentationTestCase2<MainActivity> {

public TestRobotiumMainActivity() {
    super(MainActivity.class);
}

When running tests, I came up with the following error:

java.lang.NoClassDefFoundError: com.myapplication.app.activities.MainActivity

To better understand the point, I wrote the following test:

import static org.assertj.core.api.Assertions.assertThat;

public class TestMock extends AndroidTestCase {

    //region Test methods
    public void testMethod() {
        assertThat(android.app.Activity.class).isNotNull();
        assertThat(android.support.v4.app.FragmentActivity.class).isNotNull();
        assertThat(com.myapplication.app.activities.MainActivity.class).isNotNull();
    }
    //endregion
}

And the test fails on the last line, giving the same error as above.

And when I change my MainActivity to extend android.app.Activity, the test turns to success.

Could someone suggest me a solution to solve this issue please?