example.swift

// Version A
// - crashes if viewControllers is empty
// - crashes if first VC is not of type MyViewController
let navigationController = ...
let myViewController = navigationController.viewControllers.first as! MyViewController
myViewController.configurationObject = ...

// Version B
// - doesn't crash, ever
// - does nothing if viewControllers is empty or the first is not of type MyViewController
let navigationController = ...
if let myViewController = navigationController.viewControllers.first as? MyViewController {
    myViewController.configurationObject = ...
}

// Version C
// - adds an `else` clause to version B
else {
    print("View controllers is empty or the first is not of type \(MyViewController.self)")
}