Offline document parsing not working

[Pmmoks]

I'm struggling to get py-xbrl package working in offline mode. I'm using the code snippet from the documentation, but I keep getting the error...

AttributeError: 'NoneType' object has no attribute 'attrib'

which relates to line 453 in the file instance.py

schema_uri: str = schema_ref.attrib[XLINK_NS + 'href']

Code snippet I'm using...

import logging
from xbrl.cache import HttpCache
from xbrl.instance import XbrlParser, XbrlInstance
# just to see which files are downloaded
logging.basicConfig(level=logging.INFO)

cache: HttpCache = HttpCache('./cache')
parser = XbrlParser(cache)

schema_path = "./cache/aapl-20210925/aapl-20210925.html"
inst: XbrlInstance = parser.parse_instance(schema_path)