RDF feeds not supported
Summary
It seems like RDF is not supported. I'm not sure if you plan to support this format, however there are feeds that only have this.
In the code the following is checked:
items = xml.getElementsByTagName('channel')[0].getElementsByTagName('item');
But 'item' is not nested within 'channel', it's on the same level. https://validator.w3.org/feed/docs/rss1.html
Example feed: https://nachrichtenfeeds.br.de/rdf/boards/QXAQcxl
Android version and device info
Android 12 / Nunti 1.2.0