Checked the code and it works as expected.

Correct check in GetAppIdsNoArcEnabledCheck should be 
DCHECK((GetApp(app_id.key())) || (!IsArcEnabled() && !default_apps_.HasApp(app_id.key())));

GetAppIdsNoArcEnabledCheck lists all apps, but GetApp checks into consideration if ARC is currently available. 

I tested and all cases when GetApp returns nullptr is for the case when Arc is not enabled and app is not default app.