base

Handle null result from getVirtualChildAt()

Also adds final where the method was being called, adds Nullable
annotation to method, and updates docs.

Bug: 25985497
Change-Id: I847a8507f2e3970f1340cddf4abf8650dda22b35
(cherry picked from commit ad52693cf3dfe3f5e84f5a22fe4c3787f2f2cbfb)