def sort_key(string):
    """If string is valid returns string[1][2].

    We need the 3rd char of the 2nd string tuple,
    so we need length bigger than 1
    and length of the 2nd element bigger than 2.
    Return None otherwise.
    """
    if len(string) < 2:
        return None
    if len(string[1]) < 3:
        return None
    return string[1][2]