<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def get_bibtex(identifier):
 """ Try to fetch BibTeX from a found identifier. .. note:: Calls the functions in the respective identifiers module. :param identifier: a tuple (type, identifier) with a valid type. :returns: A BibTeX string or ``None`` if an error occurred. # TODO: Should return a BiBTeX object? """

    identifier_type, identifier_id = identifier
    if identifier_type not in __valid_identifiers__:
        return None

    # Dynamically call the ``get_bibtex`` method from the associated module.
    module = sys.modules.get("libbmc.%s" % (identifier_type,), None)
    if module is None:
        return None
    return getattr(module, "get_bibtex")(identifier_id)