Hi Peter,
Thanks for looking into this. Yes, I can confirm there is a field group which explicitly defines 'id' => null (no idea how that happened). Correcting it now would probably break the site though. Since things were working fine still up until the last update, I suppose the new version has tightened the signature for sanitize_id(). I realize that this code is technically invalid, since individual field IDs are required. But Meta Box becoming fatal after an update would still warrant better defensive handling I think.
