ClassCastException on Activity on Unit Test Espresso

It's been days but I still don't understand how to successfully create a unit test on Android espresso.

I have this function that is called when the login_staff_member_btn is called.

    private fun showLoginEmail(){
        val loginEmailFragment: Fragment = LoginEmailFragment()

        (activity as MainActivity).replaceFragment(loginEmailFragment, LoginEmailFragment.TAG)
    }

I've stated the @Rule to set the activity of the test.

@RunWith(AndroidJUnit4::class)
class LoginSelectTypeFragmentTest {
    @get:Rule var activityScenarioRule = ActivityScenarioRule(MainActivity::class.java)


    @Test
    fun clickStaffMemberBtn() {
        launchFragmentInContainer<LoginSelectTypeFragment>(themeResId = R.style.SplashTheme)
        onView(withId(R.id.login_staff_member_btn)).perform(click())
    }

And I get this error

Caused by: java.lang.ClassCastException: androidx.fragment.app.testing.FragmentScenario$EmptyFragmentActivity cannot be cast to .MainActivity

It seems like the activity was not set. Any idea what I did wrong or what should be updated?

Please let me know.

Thank you in advance!