NSWindowController

If you subclass this and override func loadWindow(), you must also override var windowNibName: NSNib.Name?. Just return any (non-nil) string. It doesn't matter what. (NSViewController has no such requirement.)

A basic window controller, then, might look like:

class MyWindowController: NSWindowController {
    override var windowNibName: NSNib.Name? { return "X" }

    override func loadWindow() {
        self.window = NSWindow(contentRect: NSRect(...),
                               styleMask: [.titled, .resizable, .miniaturizable, .closable],
                               backing: .buffered,
                               defer: false)
        self.window?.title = "My Cool Window!"
    }

    override func windowDidLoad() {
        self.contentViewController = MyViewController()
    }
}