def get_file_by_id(self, file_id):
        """
        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.
        """
        file_ref = self.objects[file_id]
        if not isinstance(file_ref, PBXFileReference):
            return None
        return file_ref