DO NOT MERGE: Add a null check for the OnStartTetheringCallback.

This avoids a NullPointerException when trying to call the callback
and gives a more readable error message.

(cherry picked from commit 5f277e16)

(cherry picked from commit c8e4cc8e)

Change-Id: Ia5f328c4b25aa6624dc3c8493eb0c7bbab99904c