Returns, for a given URL, the open document whose file or file package is located by the URL, or
nil if there is no such open document.
- macOS 10.4+
The URL of the document you wish to look up.
The newly created
NSDocument object, or
nil if the document could not be created.
The default implementation of this method queries each open document to find one whose URL matches, and returns the first one whose URL does match.