SupportFragmentManager destroyed, behind the scenes

I have this scenario on my MainActivity:

 // onCreate
firebaseAuth.addAuthStateListener { firebaseAuth ->
            when (firebaseAuth.currentUser) {
                null -> {
                    hideAppBars()
                    clearBackStack(supportFragmentManager)
                    showFragment(fragment = LoginOrRegisterFragment())
                }
                else -> {
                    showAppBars()
                    clearBackStack(supportFragmentManager)
                    showFragment(fragment = HomeFragment())
                }
            }
        }

The clearBastack is just a method that is popping the full backstack of the Fragments:

private fun clearBackStack(fragmentManager: FragmentManager) {
        with(fragmentManager) {
            if (backStackEntryCount > 0)
                popBackStack()
        }
    }

And showFragment method:

fun showFragment(fragment: Fragment, addToBackStack: Boolean = false) {
            supportFragmentManager.beginTransaction().apply {
                replace(R.id.fragmentContainer, fragment)
                if (addToBackStack) addToBackStack(null)
            }.commit()
    }

In a usual flow, everything goes OK. Hit Login: BackStack clears and from LoginFragment I get to HomeFragment. However, if I press back when I'm in the LoginFragment and resume , I get IllegalStateException: FragmentManager has been destroyed

What seems to fix the issue

Explicitly checking if(!supportFragmentManager.isDestroyed):

fun showFragment(fragment: Fragment, addToBackStack: Boolean = false) {
        if (!supportFragmentManager.isDestroyed) {
            supportFragmentManager.beginTransaction().apply {
                replace(R.id.fragmentContainer, fragment)
                if (addToBackStack) addToBackStack(null)
            }.commit()
        }
    }

UPDATE: Full stacktrace:

java.lang.IllegalStateException: FragmentManager has been destroyed
        at androidx.fragment.app.FragmentManager.enqueueAction(FragmentManager.java:1725)
        at androidx.fragment.app.BackStackRecord.commitInternal(BackStackRecord.java:321)
        at androidx.fragment.app.BackStackRecord.commit(BackStackRecord.java:286)
        at com.coroutinedispatcher.datacrypt.MainActivity.showFragment(MainActivity.kt:57)
        at com.coroutinedispatcher.datacrypt.MainActivity.showFragment$default(MainActivity.kt:52)
        at com.coroutinedispatcher.datacrypt.MainActivity$onCreate$1.onAuthStateChanged(MainActivity.kt:36)
        at com.google.firebase.auth.zzj.run(com.google.firebase:firebase-auth@@19.4.0:3)
        at android.os.Handler.handleCallback(Handler.java:873)
        at android.os.Handler.dispatchMessage(Handler.java:99)
        at com.google.android.gms.internal.firebase_auth.zzj.dispatchMessage(com.google.firebase:firebase-auth@@19.4.0:6)
        at android.os.Looper.loop(Looper.java:193)
        at android.app.ActivityThread.main(ActivityThread.java:6669)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:493)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:858)

Line that throws is supportFragmentManager.apply{bla()}.commit().

Question is, why, what is actually happening in the background?

You should remove the AuthStateListnener in onDestroy of Activity.

 // onCreate
private val authStateListener = AuthStateListener { firebaseAuth ->
            when (firebaseAuth.currentUser) {
                null -> {
                    hideAppBars()
                    clearBackStack(supportFragmentManager)
                    showFragment(fragment = LoginOrRegisterFragment())
                }
                else -> {
                    showAppBars()
                    clearBackStack(supportFragmentManager)
                    showFragment(fragment = HomeFragment())
                }
            }
        }

override fun onCreate(...) {
    super.onCreate(...)
    firebaseAuth.addAuthStateListener(authStateListener)
}


override fun onDestroy() {
    firebaseAuth.removeAuthStateListener(authStateListener)
    super.onDestroy()
}

Although technically you should also consider that this could still trigger fragment transactions after onStop, which would cause a this action cannot be performed after onSaveInstanceState error, so you should actually only handle the navigation action if the Activity is at least started.

You could use https://github.com/Zhuinden/live-event for example for that.