Unit test when we have lazy loading of LiveData in ViewModel

Question

We have following code for ViewModel :

class MyViewModel : ViewModel() {
    private val users: MutableLiveData<List<User>> by lazy {
        MutableLiveData().also {
            loadUsers()
        }
    }

    fun getUsers(): LiveData<List<User>> {
        return users
    }

    private fun loadUsers() {
        // Do an asynchronous operation to fetch users.
    }
}

I want to have a unit test for this code :

    @Test
    fun loadUsers() {
        val user= User("name")

    val observableResponse =
            Observable.just(api.UserWrapper(Lists.newArrayList(user)))
    `when`(api.loadUsers()).thenReturn(observableResponse)

    with(viewModel) {
        with(this.users.value!!) {
            assertFalse(isEmpty())
            assertTrue(size == 1)
        }
    }
}

But it does not passing the test. It is due to lazy loading :

private val users: MutableLiveData<List<User>> by lazy {
        MutableLiveData().also {
            loadUsers()
        }
    }

What could be the solution?

Here is the error :

NullPointerException
    at java.security.AccessController.doPrivileged(Native Method)
    at java.net.URLClassLoader.findClass(URLClassLoader.java:362)
    at java.lang.ClassLoader.loadClass(ClassLoader.java:424)
    at java.lang.ClassLoader.loadClass(ClassLoader.java:411)
    at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:349)
    at java.lang.ClassLoader.loadClass(ClassLoader.java:357)
    at io.reactivex.Observable.subscribe(Observable.java:12034)
    at io.reactivex.Observable.subscribe(Observable.java:12016)
    at io.reactivex.Observable.subscribe(Observable.java:11945)
    at com.sample.android.tmdb.ui.detail.DetailViewModel$_cast$2.invoke(DetailViewModel.kt:25)
    at com.sample.android.tmdb.ui.detail.DetailViewModel$_cast$2.invoke(DetailViewModel.kt:12)
    at kotlin.SynchronizedLazyImpl.getValue(LazyJVM.kt:74)
    at com.sample.android.tmdb.ui.detail.DetailViewModel.get_cast(DetailViewModel.kt)
    at com.sample.android.tmdb.ui.detail.DetailViewModel.access$get_cast$p(DetailViewModel.kt:12)
    at com.sample.android.tmdb.ui.detail.DetailViewModel$_cast$2$$special$$inlined$also$lambda$4.accept(DetailViewModel.kt:26)
    at com.sample.android.tmdb.ui.detail.DetailViewModel$_cast$2$$special$$inlined$also$lambda$4.accept(DetailViewModel.kt:12)