UINavigationController is nil in swift 3

In my main storyboard I have 1 Navigation controller and subsequent swift file NavigationCtr.swift.

The viewController is in different xib.

Now I want to push my viewController from the NavigationCtr class.

       let vcFirst = FirstViewController(nibName: "FirstViewController", bundle: nil)
    self.navigationController!.pushViewController(vcFirst, animated: true)

I am getting a exception

fatal error: unexpectedly found nil while unwrapping an Optional value

So when I trying to print from viewDidLoad

   print(self.navigationController)

in NavigationCtr.swift class it is giving nil. So nothing works

I created a new project in objective C and it works fine. Attached the storyboard image Any hint in the direct direction is highly appreciated. Note : - I am new to swift