Fix NPE

Test: NotificationChannelGroupTest
Test: view notification settings for an app that doesn't use groups
Fixes: 244574602
Bug: 241764350
Bug: 241764340
Bug: 241764135
Bug: 242702935
Bug: 242703118
Bug: 242703202
Bug: 242702851
Bug: 242703217
Bug: 242703556
Change-Id: I9c681106f6d645e62b0e44903d40aa523fee0e95