The error:

java.lang.NullPointerException
	at androidx.compose.ui.layout.SubcomposeLayoutState.disposeCurrentNodes$ui_release(SubcomposeLayout.kt:386)

Here are the Composable functions being rendered (and tested) in this example:

Home Screen

@Composable
fun HomeScreen(
    viewModel: HomeViewModel = hiltViewModel(),
    onNavigate: (destination: NavigationDestination) -> Unit = {}
) {
    if (viewModel.isAuthenticated) {
        HomeScreen()
    } else {
        LaunchedEffect(viewModel.isAuthenticated) {
            onNavigate(NavigationDestination.Root.Onboarding)
        }
    }
}

@Composable
fun HomeScreen() {
    val navController = rememberNavController()
    val navBackStackEntry by navController.currentBackStackEntryAsState()
    val currentDestination = navBackStackEntry?.destination
    Scaffold(
        bottomBar = {
            BottomAppBar(
                modifier = Modifier,
                backgroundColor = MaterialColors.background
            ) {
                BottomNav(currentDestination) { screen ->
                    navController.navigate(screen.route)
                }
            }
        },
        modifier = Modifier.semantics { testDescription = "Home Screen" }
    ) { padding ->
        HomeNavHost(
            navController = navController,
            modifier = Modifier.padding(padding)
        )
    }
}

HomeNavHost

@Composable
fun HomeNavHost(navController: NavHostController, modifier: Modifier = Modifier) {
    NavHost(
        navController = navController,
        startDestination = NavigationDestination.Home.Coach.route,
        modifier = modifier
    ) {
        composable(NavigationDestination.Home.Coach.route) {
            CoachScreen()
        }
        composable(NavigationDestination.Home.Discover.route) {
            DiscoverScreen()
        }
        composable(NavigationDestination.Home.Library.route) {
            LibraryScreen()
        }
        composable(NavigationDestination.Home.You.route) {
            YouScreen()
        }
        composable(NavigationDestination.Home.Search.route) {
            SearchScreen()
        }
    }
}

I have two tests,

HomeNavHostTest

, for testing the NavHost in isolation, and

HomeScreenTest

. The crash occurs when

HomeScreenTest

runs after

HomeNavHostTest

HomeNavHostTest

@HiltAndroidTest
class HomeNavHostTest {

    @get:Rule(order = 0)
    var hiltRule = HiltAndroidRule(this)

    @get:Rule(order = 1)
    val composeTestRule = createAndroidComposeRule<MainActivity>()

    private lateinit var navController: NavHostController

    @Before
    fun setup() {
        hiltRule.inject()

        composeTestRule.setContent {
            navController = rememberNavController()
            HomeNavHost(navController = navController)
        }
    }

    @Test
    fun initialDestinationIsCoach() {
        assert(navController.currentDestination?.route == NavigationDestination.Home.Coach.route)
    }

{

HomeScreenTest

@ExperimentalCoroutinesApi
@HiltAndroidTest
class HomeScreenTest {

    @get:Rule(order = 0)
    var hiltRule = HiltAndroidRule(this)

    @get:Rule(order = 1)
    val composeTestRule = createAndroidComposeRule<MainActivity>()

    @Inject
    lateinit var userManager: FakeUserManager

    private var navigationDestination: NavigationDestination? = null

    @Before
    fun setup() {
        hiltRule.inject()
    }

    fun renderScreen(){
        composeTestRule.setContent {
            HomeScreen() {
                navigationDestination = it
            }
        }
    }

    @Test
    fun bottomTabsAreDisplayed() {
        // When the authentication state is 'Authenticated'
        userManager.onIsAuthenticated = { true }

        renderScreen()

        // Then the bottom tabs are displayed
        composeTestRule.onNodeWithTestDescription("coach").assertIsDisplayed()
        composeTestRule.onNodeWithTestDescription("discover").assertIsDisplayed()
        composeTestRule.onNodeWithTestDescription("library").assertIsDisplayed()
        composeTestRule.onNodeWithTestDescription("you").assertIsDisplayed()
        composeTestRule.onNodeWithTestDescription("search").assertIsDisplayed()
    }
}

Full StackTrace:

java.lang.NullPointerException
	at androidx.compose.ui.layout.SubcomposeLayoutState.disposeCurrentNodes$ui_release(SubcomposeLayout.kt:386)
	at androidx.compose.ui.layout.SubcomposeLayoutKt$SubcomposeLayout$3$invoke$$inlined$onDispose$1.dispose(Effects.kt:484)
	at androidx.compose.runtime.DisposableEffectImpl.onForgotten(Effects.kt:85)
	at androidx.compose.runtime.CompositionImpl$RememberEventDispatcher.dispatchRememberObservers(Composition.kt:793)
	at androidx.compose.runtime.CompositionImpl.dispose(Composition.kt:496)
	at androidx.compose.ui.platform.WrappedComposition.dispose(Wrapper.android.kt:171)
	at androidx.compose.ui.platform.WrappedComposition.onStateChanged(Wrapper.android.kt:179)
	at androidx.lifecycle.LifecycleRegistry$ObserverWithState.dispatchEvent(LifecycleRegistry.java:354)
	at androidx.lifecycle.LifecycleRegistry.backwardPass(LifecycleRegistry.java:284)
	at androidx.lifecycle.LifecycleRegistry.sync(LifecycleRegistry.java:302)
	at androidx.lifecycle.LifecycleRegistry.moveToState(LifecycleRegistry.java:148)
	at androidx.lifecycle.LifecycleRegistry.handleLifecycleEvent(LifecycleRegistry.java:134)
	at androidx.lifecycle.ReportFragment.dispatch(ReportFragment.java:68)
	at androidx.lifecycle.ReportFragment$LifecycleCallbacks.onActivityPreDestroyed(ReportFragment.java:224)
	at android.app.Activity.dispatchActivityPreDestroyed(Activity.java:1516)
	at android.app.Activity.performDestroy(Activity.java:8312)
	at android.app.Instrumentation.callActivityOnDestroy(Instrumentation.java:1364)
	at androidx.test.runner.MonitoringInstrumentation.callActivityOnDestroy(MonitoringInstrumentation.java:756)
	at android.app.ActivityThread.performDestroyActivity(ActivityThread.java:5374)
	at android.app.ActivityThread.handleDestroyActivity(ActivityThread.java:5420)
	at android.app.servertransaction.DestroyActivityItem.execute(DestroyActivityItem.java:47)
	at android.app.servertransaction.ActivityTransactionItem.execute(ActivityTransactionItem.java:45)
	at android.app.servertransaction.TransactionExecutor.executeLifecycleState(TransactionExecutor.java:176)
	at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:97)
	at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2210)
	at android.os.Handler.dispatchMessage(Handler.java:106)
	at android.os.Looper.loopOnce(Looper.java:201)
	at android.os.Looper.loop(Looper.java:288)
	at android.app.ActivityThread.main(ActivityThread.java:7839)
	at java.lang.reflect.Method.invoke(Native Method)
	at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:548)
	at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1003)

Perhaps it’s unusual to have two sets of tests that execute parts of the same Composable, and there’s an issue with the

ViewModel

not being recreated or something?

OK, so I think I found the cause of the problem (unconfirmed, but likely), and it seems to be my mistake (not surprised!) I believe the problem occurs due to multiple calls to 

MainActivity.setContent()

 during testing, and is probably just user error. Because I’m using Hilt, and I need to inject dependencies in test, I need the tests to run against a 

@AndroidEntryPoint

 annotated Activity. So, I decided to just use my existing 

MainActivity

@get:Rule(order = 1)
val composeTestRule = createAndroidComposeRule<MainActivity>()

@AndroidEntryPoint
class MainActivity : ComponentActivity() {

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)

        setContent {
            HomeScreen()
        }
    }
}

Then, during the Composable test, I need to set some ViewModel state, before calling `setContent()`:

@Test
    fun when_undetermined_then_showLoadingScreen() {
        // When the authentication state is 'Unauthenticated'
        userManager.onUpdateAuthenticationState = { UserAuthenticationState.Undetermined }

        composeTestRule.setContent {
            HomeScreen() {
                navigationDestination = it
            }
        }

        // Then the initial screen is 'loading'
        composeTestRule
            .onNodeWithTestDescription("Loading Screen")
            .assertIsDisplayed()
    }

The problem is, 

MainActivity

 already calls 

setContent()

. And the 

createAndroidComposeRule

 starts the 

MainActivity

. Then, during the test, 

setContent()

 is called again. I’m not quite sure what the implications are of this, when combined with HiltViewModel and Singleton dependencies, but it’s not that surprising that the tests are tripping over themselves.

tl;dr Tests run against

MainActivity

, which calls

setContent()

and the test also calls

setContent()

which leads to a bunch of weird bugs