1. Don't create UiBlockerController if there is no related controller
2. Don't update visibility if UiBlockerController is null
3. Use findPreference() from DashboardFragment, which already has null
check.
Change-Id: Iee24c64317fb9d5a1cf2076d25728af485d390c5
Fixes: 122807414
Fixes: 122805831
Test: RunSettingsRoboTests