Fix NPE that can occur on uiMode change if a group has a single child.

Note that the ternary null check inside init is not necessary given the other change, but seems prudent.

Fixes: 173777455
Fixes: 173650635
Test: manual -- use go/notify-apk to post a group of 2, dismiss 1, change font size.  Observe SystemUI no longer crashes.
Change-Id: I71a820a1ed2ac7ec71c4d9f78855ff264c5d5051