Add null check for handler on Clock view

On reboot, we've encountered an issue where the handler returns null,
causing a null pointer exception in the broadcast receiver.

In this CL, we add a null check and an error message to prevent a sysui
crash.

Bug: 148869042
Test: build, reboot a few times
Change-Id: Iac7f3a538be9a53d4a76926523aa2a3f4f22723d
Merged-In: Ie8a2627004876cf35291d52bd3686a5014498f52