from typing import List
from typing import Optional


def _get_next(next_urls: List[str]) -> Optional[str]:
    """
    Extract the next URL from the list of candidates

    :param List[str] next_urls: List of URLs to check
    :rtype: Optional[str]
    :return: A URL or None if invalid
    """
    if len(next_urls) < 1:
        return None
    next_url = next_urls[0]
    if next_url is None:
        return None
    next_url = next_url.strip()
    if next_url.startswith('/'):
        return next_url
    return None