When using 'bestvideo/best,bestaudio', 'bestvideo/best' must be set as the current_selector (instead of appending it to the selectors), otherwise when it gets the ',' it would append 'None' to the selectors.
'bestvideo+bestaudio/best' was incorrectly interpreted as 'bestvideo+(bestaudio/best)', so it would fail if 'bestaudio' doesn't exist instead of falling back to 'best'.