In a window with more than one Tab in TabLevel 0 the code tries to create a RecentItems record always from Tab 0 if you save a detail tab (or another TabLevel 0 tab).
If your first Tab does not contain any record you are not even able to save the record. The error message is like "FillMandatory: Record_ID is mandatory".
My usecase is a window that shows my active Requests in the first tab and all Requests in the second. If there are no active Requests and I try to change one of the Requests in the second tab I raise the given error message.