The most appropriate window (of the windows associated with the document) to use as the parent window of a document-modal sheet.
- macOS 10.3+
The value of this property may be
nil, in which case the sender should present an app-modal panel. The
NSDocument implementation of this property sets the value to the window of the first window controller, or
[NSApp main if there are no window controllers or if the first window controller has no window.