IssueTracker

Component used: Navigation
Version used: 2.1.0-beta02
Devices/Android versions reproduced on:

When using sharedElementTransition from Fragment A to Fragment B, we call Fragment#postponeEnterTransition first at Fragment B onCreateView.
Then if we press back button or navigate to other fragment before Fragment#startPostponedEnterTransition of Fragment B was called, the following exception will happen when trying to navigate to other fragments.

StackTrace:
E/AndroidRuntime: FATAL EXCEPTION: main
    Process: com.example.fragmenttransitionbug, PID: 8900
    java.lang.NullPointerException: Attempt to invoke virtual method 'boolean androidx.fragment.app.FragmentManagerImpl.isPrimaryNavigation(androidx.fragment.app.Fragment)' on a null object reference
        at androidx.fragment.app.Fragment.performPrimaryNavigationFragmentChanged(Fragment.java:2662)
        at androidx.fragment.app.FragmentManagerImpl.dispatchParentPrimaryNavigationFragmentChanged(FragmentManagerImpl.java:2805)
        at androidx.fragment.app.FragmentManagerImpl.setPrimaryNavigationFragment(FragmentManagerImpl.java:2800)
        at androidx.fragment.app.BackStackRecord.executeOps(BackStackRecord.java:422)
        at androidx.fragment.app.FragmentManagerImpl.completeExecute(FragmentManagerImpl.java:1989)
        at androidx.fragment.app.FragmentManagerImpl$StartEnterTransitionListener.completeTransaction(FragmentManagerImpl.java:3379)
        at androidx.fragment.app.FragmentManagerImpl.executePostponedTransaction(FragmentManagerImpl.java:1769)
        at androidx.fragment.app.FragmentManagerImpl.removeRedundantOperationsAndExecute(FragmentManagerImpl.java:1800)
        at androidx.fragment.app.FragmentManagerImpl.execPendingActions(FragmentManagerImpl.java:1727)
        at androidx.fragment.app.FragmentManagerImpl$2.run(FragmentManagerImpl.java:150)
        at android.os.Handler.handleCallback(Handler.java:873)
        at android.os.Handler.dispatchMessage(Handler.java:99)
        at android.os.Looper.loop(Looper.java:193)
        at android.app.ActivityThread.main(ActivityThread.java:6669)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:493)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:858)


Is this the correct behavior?
Since it's a common pattern when using Glide to load a remote image with sharedElementTransition.
We want to call Fragment#startPostponedEnterTransition after the remote image was ready to show.
But the loading can take some time which cause the above situation to happen.
A sample project was attached.

If this is a bug in the library, we would appreciate if you could attach:
- Sample project to trigger the issue.
- A screenrecord or screenshots showing the issue (if UI related).