IssueTracker

Please include a sample project that reproduces your issue.

Thanks for the sample app, I was able to reproduce your issue.

You can work around it by switching the order that you call replace and setPrimaryNavigationFragment and call setPrimaryNavigationFragment *after* replace:

    requireFragmentManager().beginTransaction()
        .addToBackStack(null)
        .replace(Window.ID_ANDROID_CONTENT, fragment)
        .setPrimaryNavigationFragment(fragment)
        .commit()

Project: platform/frameworks/support
Branch: androidx-master-dev

commit a4806ba69d611dcc36b794870e9dc80981371ec9
Author: Ian Lake <ilake@google.com>
Date:   Tue Jun 11 13:14:42 2019

    Fix pop when calling setPrimary before replace
   
    When popping a FragmentTransaction, the operations
    you apply are undone in reverse ordering. This means
    that when you call setPrimaryNavigationFragment()
    before replace(), the replace() is undone first, causing
    the current Fragment to be removed from the
    FragmentManager before the code for undoing
    setPrimaryNavigationFragment runs.
   
    We now ensure that performPrimaryNavigationFragmentChanged
    is only called if the Fragment is still attached.
   
    Test: new replacePrimaryNavAfterSetPrimary test passes
    Fixes: 134673465
    Change-Id: I1a126c265c2350ed4e7d437fb68df29376ff5fcf

M       fragment/fragment/src/androidTest/java/androidx/fragment/app/PrimaryNavFragmentTest.kt
M       fragment/fragment/src/main/java/androidx/fragment/app/FragmentManagerImpl.java

https://android-review.googlesource.com/980591
https://goto.google.com/android-sha1/a4806ba69d611dcc36b794870e9dc80981371ec9

This is fixed internally and will be in the next release of Fragments.