Consider this absolutely simple looking NSWindowController + NSViewController pair:

@objc final class MyFabWC: NSWindowController {
  @objc static let shared = MyFabWC(window: NSWindow(contentViewController: MyFabVC()))
  
  override func windowDidLoad() {
    super.windowDidLoad()
    
    // Implement this method to handle any initialization after your window controller's window has been loaded from its nib file.
  }
  
}

@objc final class MyFabVC: NSViewController {
  override func loadView() {
    view = NSView()
  }
}

Now if I try to preview this using Xcode Previews like so:

#if DEBUG
@available(macOS 14.0, *)
#Preview("GroupEmailVC") {
  MyFabVC()
}
#endif

So consequentially, the NSWindowController isn't of any immediate effect here, but I've included it for completness.

When previewing the above, I get the following error:

== PREVIEW UPDATE ERROR:

FailedToAnalyzeBuiltTargetDescription: Could not analyze the built target description for MyFabApp to create the preview.

WHY!? Why can it not analyze the built target? The target builds just fine with a few warnings, but no build errors.