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)

Change-Id: Ia419ff68ef10f308f9e44be47420e27099ee6070
1 file changed