"Consistency issue: no parent for object" error on a valid project

Sample project: [RNCAsyncStorage](https://github.com/react-native-community/async-storage/tree/7042363674fd22aa9b4cb12672d6ed6f06b3b13e/ios)
Opening it with Xcodeproj and saving it without any changes results to an error:
```
RuntimeError: [Xcodeproj] Consistency issue: no parent for object `RNCAsyncStorage.h`: `HeadersBuildPhase`, `CopyFilesBuildPhase`
from gems/xcodeproj-1.9.0/lib/xcodeproj/project/object/helpers/groupable_helper.rb:18:in `parent'
from gems/xcodeproj-1.9.0/lib/xcodeproj/project/object/build_file.rb:63:in `ascii_plist_annotation'
from gems/xcodeproj-1.9.0/lib/xcodeproj/project.rb:296:in `block in to_ascii_plist'
from gems/xcodeproj-1.9.0/lib/xcodeproj/project.rb:295:in `each'
from gems/xcodeproj-1.9.0/lib/xcodeproj/project.rb:295:in `to_ascii_plist'
from gems/xcodeproj-1.9.0/lib/xcodeproj/project.rb:364:in `block in save'
from gems/atomos-0.1.3/lib/atomos.rb:21:in `block in atomic_write'
from lib/ruby/2.4.0/tempfile.rb:295:in `open'
from gems/atomos-0.1.3/lib/atomos.rb:17:in `atomic_write'
from gems/xcodeproj-1.9.0/lib/xcodeproj/project.rb:363:in `save'
```

Still, the project itself is a valid Xcode project that works fine.
Looks like the problem is related to sources that aren't added to any PBXGroup. Maybe we can add some fallback to `ascii_plist_annotation` since this annotation is just a comment? Having no parent group doesn't seem to be illegal from Xcode's point of view.