The current implementation might lead to infinite loops.

I think that is possible to add any kind of group (for example folders linked to a path).

This is an attribute because a it is usually a reference to the root object or to a file reference to another project. The reference to the root object causes a retain cycle that could cause issues (e.g. in to_tree_hash). Usually those objects are retained by at least another object (the {Project} for the root object and a {PBXGroup} for the reference to another project) and so the referenced object should be serialized. If this assumption is incorrect, there could be loss of information opening and saving an existing project.