无法转换“Range<String”类型的值。索引>'到预期的参数类型“uitextange”

class ViewController: UIViewController {
  @IBOutlet weak var textView: UITextView!
  let firstString: String = "xxx"
  let secondString: String = "yyy"

  override func viewDidLoad() {
    super.viewDidLoad()
    textView.text = firstString
  }

  func replace() {
    var finalString: String?
    let range = firstString.startIndex..<firstString.endIndex
    print(firstString[range])
    textView.replace(range, withText: secondString)
  }

  @IBAction func replaceButton(_ sender: Any) {
    replace()
  }
}