Add rightBarButtonItems to NavBar in UISplitViewController (swift 3)

Question

I am trying to set a custom BarButtonItem to my NavigationBar of my DetailsVC in my UISplitViewController with these two lines of code:

let barbuttonitem = UIBarButtonItem(title: "Hi", style: .plain, target: nil, action: nil)
    navigationItem.rightBarButtonItems = [barbuttonitem]

but somehow nothing happens. When i create a new project and add these 2 lines of codes for my navBar it works, but in my real App it doesn't.

Thanks for your help