Add Null checks while processing pairing cancel intent

Use case: Exception while handling pairing cancel
          Intent received
1. Initiate pairing from DUT to remote
2. Power off DUT after getting pairing popup

Root Cause:Crash observed in settings application

Change-Id: Ia05fe196ecf6a1d4ec26f85b0cf984e23089c728
Fix: Fixed by adding Null check in PairingCancelHandler