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 5f277e1667b49976a77c9fa79b9a04d775f3b49e)

(cherry picked from commit c8e4cc8e9919260dac5f2f8a76b693e8a29bfcc1)

Change-Id: Ia5f328c4b25aa6624dc3c8493eb0c7bbab99904c
1 file changed