Prevent NPE when checking focused stage before root task is ready

If the root task of the stage is not created, treat the stage as not
focused to prevent trowing NPE when traversing the root task and its
child.

Fix: 199060332
Test: manual check isFocused won't throw before root task is ready
Change-Id: I3869f934b641c8ffd77730452d0501d65063785c