The definition of
Return a private FragmentManager for placing and managing Fragments
inside of this Fragment.
Meanwhile the definition of
getFragmentManager() (or in this case
Return the FragmentManager for interacting with fragments associated
with this fragment’s activity.
Basically, the difference is that Fragment’s now have their own internal
FragmentManager that can handle Fragments. The child FragmentManager is the one that handles Fragments contained within only the Fragment that it was added to. The other FragmentManager is contained within the entire
In this case, what I’m guessing is you’ve added the Fragments to the Activity’s FragmentManager. You get the child FragmentManager which doesn’t contain what you are looking for. Thus you get the exception because it can’t find the Fragment with the given ID because it’s in a different FragmentManager.