IssueTracker

Component used: Fragment
Version used: 1.1.0-beta01

For any given fragment backstack, if the fragment to be popped off the backstack is the current primary navigation fragment, this results in a NullPointerException on that popped fragment's FragmentManager receiving notification that the primary navigation fragment changed. Because that fragment is being removed, it no longer has a host nor a reference to that host's FragmentManager. Specifically, this happens at androidx.fragment.app.Fragment.performPrimaryNavigationFragmentChanged, line 2662 (mFragmentManager is null).

Example:

fragmentManager.beginTransaction()
    .add(containerId, FragmentA())
    .commit()

Current backstack:
A (not primary)

Transaction adding a new fragment that is to be a primary nav fragment:

val fragmentB = FragmentB()
fragmentManager.beginTransaction()
    .addToBackStack(null)
    .setPrimaryNavigationFragment(fragmentB)
    .replace(containerId, fragmentB)
    .commit()

Current backstack:
A -> B (primary)

If activity.onBackPress() or fragmentManager.popBackStack() is called, as B is being popped, the FragmentManager's primary nav fragment is set to null, and B (that is now removed) notifies its host FragmentManager of this change (which B no longer has a host because it was removed).