You must call setGraph() before calling getGraph()

You'll get this same error if you are like me and forget to add to your FragmentContainerView

app:navGraph="@navigation/nav_graph"
app:defaultNavHost="true"

This is fixed as per the 2.2.0-rc03 release notes:

NavHostFragment now correctly restores the graph after a configuration change when used with FragmentContainerView. (b/143752103)

So make sure you are using 2.2.0-rc03.