Avoid NPE while constructing TimerItemFragment

Bug: 27417914

Monkey testing has produced an NPE in TimerItemFragment.onCreateView.
The NPE indicates that a Timer was removed before the corresponding
TimerItemFragment could be fully created. This would be exceedingly
difficult for a human to reproduce, but the fix is a trivial null
check, so this CL contains that change.

Change-Id: I0e0911e9111c1c5a6cd7b27cdfc547a8acb1cd8f