code: def get_file_by_id(self, file_id):
          
          file_ref = self.objects[file_id]
          if not isinstance(file_ref, PBXFileReference):
              return None
          return file_ref
docs: Gets the PBXFileReference to the given id
          :param file_id: Identifier of the PBXFileReference to be retrieved.
          :return: A PBXFileReference if the id is found, None otherwise.