Inconsistent View Hierarchy - cocoa

Question

I try to add a .xib based custom view into another .xib based custom view.
The result is looking like this:
for sub in v.subviews {
Swift.print(v.subviews) // returns array [sub]
Swift.print(sub) // returns sub
Swift.print(sub.superview) // return nil!
}
How can a view be in superview's subviews, but the superview property not correctly set? Can this happen during de/coding? What do I need to set in order for this to be correct?
The next question would be, why sub is shown correctly in "View Debugging" but not in when I need it during run time.
EDIT: (thanks Matt for looking into this)
My Code looks like this:
import AppKit
func showXIBDefinedInPanel(name: String, title: String ) {
if let w = loadXIBDefined(name: name) {
let c = NSViewController()
c.view = w
let window = NSPanel(contentViewController: c)
window.isMovable = true
window.collectionBehavior = .canJoinAllSpaces
window.tabbingMode = .disallowed
window.title = title
window.styleMask = [ .titled, .utilityWindow, .closable]
window.makeKeyAndOrderFront(w)
}
}
func loadXIBDefined(name: String) -> XIBDefined? {
var topLevelObjects : NSArray?
var result : XIBDefined? = nil
if Bundle.main.loadNibNamed(NSNib.Name(rawValue: name), owner: nil, topLevelObjects: &topLevelObjects) {
result = topLevelObjects!.first(where: { $0 is XIBDefined }) as? XIBDefined
}
return result
}
///used to embed a XIBDefined into another XIB
#IBDesignable class XIBEmbedder : NSView {
// Our custom view from the XIB file
var view: NSView!
var xibName: String!
override init(frame frameRect: NSRect) {
super.init(frame: frameRect)
view = loadXIBDefined(name: xibName)
addSubview(view)
self.frame = view.frame
}
required init?(coder decoder: NSCoder) {
super.init(coder: decoder)
view = loadXIBDefined(name: xibName)
addSubview(view)
self.frame = view.frame
}
init(name: String) {
self.xibName = name
super.init(frame: NSZeroRect)
view = loadXIBDefined(name: xibName)
addSubview(view)
self.frame = view.frame
}
}
///used as class for XIB based Custom Views
#IBDesignable class XIBDefined: NSView {
///I had an issue with an oddly moved frame, so I hard coded a fix
override func setFrameOrigin(_ newOrigin: NSPoint) {
super.setFrameOrigin(NSZeroPoint)
needsDisplay = true
}
}
#IBDesignable class WelcomeCMapView : XIBEmbedder {
init() {
super.init(name: "Welcome Concept Maps")
}
required init?(coder decoder: NSCoder) {
super.init(name: "Welcome Concept Maps")
}
}
Key are the two classes XIBDefined and XIBEmbedder. Former one can be loaded from a XIB, the latter can be used in an XIB. Therefore the later embedded XIB uses XIBDefined as the custom class, the embedding XIB has a WelcomeCMapView as custom class.
The problem-showing code on top is part of the post-processing, which is executed within viewDidLoad() from the NSViewController loading the embedding XIB.