Xcode 11: cannot instantiate view controller from storyboard with identifier

You should do following:

let viewController = storyboard.instantiateViewController(withIdentifier: "TabBarController")

parameter should be -withIdentifier- not -identifier-

You need to use

storyboard.instantiateViewController(withIdentifier: "TabBarController")

the new instantiateViewController(identifier: "TabBarController") is only available on iOS 13 and returns ViewController instead of UIViewController as you can see here