Returns the document window to use as the parent of a document-modal sheet.
- macOS 10.3+
This method searches the document's window controllers for the most suitable window to use when displaying the sheet.
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.