I have this sime line in my code:
java.lang.NullPointerException: Attempt to invoke virtual method 'android.os.Handler android.support.v4.app.FragmentHostCallback.getHandler()' on a null object reference
It turned out that there was an error in my code.
I can't show an example because there were a bunch of issues with my Activity-Fragment architecture, but the main issue was that getChildFragmentManager was actually empty and it was being called from the wrong fragment.
This was happening on a tablet (dual pane layout) where there were 4+ fragments being alive at the same time. This was being called on the wrong fragment.
So if you are experiencing a similar issue, you might have multiple instances of the same fragment in memory. Make sure that you are calling this method from the correct instance.