Why does `addTextField`'s `configuration` closure parameter is `Optional`?

[Skogetroll]

The code is:

open func addTextField(_ configuration: (_ textField: UITextField?) -> ()) {
    let textField = EMAlertTextField()
    textField.delegate = self
    configuration(textField)
    addTextField(textField)
}

EMAlertTextField.init is not optional constructor. So textField guaranteed to be non-optional.

There's no way call like configuration(nil) ever happen.

So why does it still declared as configuration: (_ textField: UITextField?) -> ()?