Same view with valid reference which becoming nil when referenced from delegate

Question

The func yesBtSetTextTo prints not nil when called from the grey button in the pink area, but the same func print nil when is called from the yellow button. I am lost on this one.

My main veiwController.swift does not have a prepareForSegue since I did not thing it is needed here.

The func in question is at the end of this code block,

MainBtVC.swift

protocol MainBtDelegate {
func yesBtSetTextTo(name: String)
}
class MainBtVC: UIViewController, MainBtDelegate {

@IBOutlet weak var yesBt: UIButton!

override func viewDidLoad() {
    super.viewDidLoad()
}

override func didReceiveMemoryWarning() {
    super.didReceiveMemoryWarning()
}

func yesBtSetTextTo(name: String) {
    print("called")
    if yesBt == nil {
        print("button is nil")
    } else {
        print("button not nil")
    }
    //      yesBt.setTitle("Exit", forState: UIControlState.Normal)
}

@IBAction func yesBtTapped(sender: AnyObject) {
    yesBtSetTextTo("dummy")
}

TopVC.swift

class TopVC: UIViewController {
var delegate: MainBtDelegate?

override func viewDidLoad() {
    super.viewDidLoad()
    delegate = MainBtVC()
}

override func didReceiveMemoryWarning() {
    super.didReceiveMemoryWarning()
}

@IBAction func buttonTapped(sender: AnyObject) {
    delegate?.yesBtSetTextTo("Exit")
}