In the example app included below I have a nested graph representing a setup workflow, on conclusion of which the download page must open. The download page can also be opened directly.

If I have an action that links from the subgraph to the download page that pops the subgraph then no crashes, but clicking the back button takes you back to the main screen, not "page 3" as is wanted.

If I don't pop the subgraph then it crashes both when I navigate to main, and when I hit the back button (the one on the device, not the toolbar). The toolbar back button takes you back to page3 (as expected).

Stack trace:
```
E/AndroidRuntime: FATAL EXCEPTION: main
java.lang.IllegalArgumentException: Navigator androidx.navigation.fragment.FragmentNavigator@ae77e1d8 reported navigation to unknown destination id com.example.myapplication:id/page3Fragment
at androidx.navigation.NavController$2.onNavigatorNavigated(NavController.java:98)
at androidx.navigation.Navigator.dispatchOnNavigatorNavigated(Navigator.java:176)
at androidx.navigation.fragment.FragmentNavigator$1.onBackStackChanged(FragmentNavigator.java:72)
at android.support.v4.app.FragmentManagerImpl.reportBackStackChanged(FragmentManager.java:2704)
at android.support.v4.app.FragmentManagerImpl.executeOpsTogether(FragmentManager.java:2411)
at android.support.v4.app.FragmentManagerImpl.removeRedundantOperationsAndExecute(FragmentManager.java:2338)
at android.support.v4.app.FragmentManagerImpl.popBackStackImmediate(FragmentManager.java:854)
at android.support.v4.app.FragmentManagerImpl.popBackStackImmediate(FragmentManager.java:797)
at android.support.v4.app.FragmentManagerImpl.popBackStackImmediate(FragmentManager.java:844)
at android.support.v4.app.FragmentManagerImpl.popBackStackImmediate(FragmentManager.java:797)
at android.support.v4.app.FragmentActivity.onBackPressed(FragmentActivity.java:186)
at android.app.Activity.onKeyUp(Activity.java:2145)
at android.view.KeyEvent.dispatch(KeyEvent.java:2633)
at android.app.Activity.dispatchKeyEvent(Activity.java:2375)
at android.support.v7.app.AppCompatActivity.dispatchKeyEvent(AppCompatActivity.java:534)
at android.support.v7.view.WindowCallbackWrapper.dispatchKeyEvent(WindowCallbackWrapper.java:58)
at android.support.v7.app.AppCompatDelegateImplBase$AppCompatWindowCallbackBase.dispatchKeyEvent(AppCompatDelegateImplBase.java:316)
at android.support.v7.view.WindowCallbackWrapper.dispatchKeyEvent(WindowCallbackWrapper.java:58)
at com.android.internal.policy.impl.PhoneWindow$DecorView.dispatchKeyEvent(PhoneWindow.java:1847)
at android.view.ViewRootImpl.deliverKeyEventPostIme(ViewRootImpl.java:3701)
at android.view.ViewRootImpl.handleImeFinishedEvent(ViewRootImpl.java:3651)
at android.view.ViewRootImpl$ViewRootHandler.handleMessage(ViewRootImpl.java:2818)
at android.os.Handler.dispatchMessage(Handler.java:99)
at android.os.Looper.loop(Looper.java:137)
at android.app.ActivityThread.main(ActivityThread.java:5041)
at java.lang.reflect.Method.invokeNative(Native Method)
at java.lang.reflect.Method.invoke(Method.java:511)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:793)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:560)
at dalvik.system.NativeStart.main(Native Method)
```

Component used:
android.arch.navigation:navigation-fragment-ktx

Version used:
1.0.0-alpha01

Devices/Android versions reproduced on:
All tested

- Sample project to trigger the issue.
See attached

- A screen record or screenshots showing the issue (if UI related).