Swift-如何使用becomeFirstResponder跳到顶部

以下是我想做的，当我点击我的button时，我想跳到顶部，但当我点击button时，我的应用程序崩溃了。

@IBAction func addToCart(_ sender: UIButton) {
if quantitylbl.text == "0" {
dismiss(animated: true)
}
// Save in to userdefault
let section = sender.tag / 100
let row = sender.tag % 100
let dict = arrayCategory[section] as? NSDictionary
let subLimit = getStringValueFromAPIValue(apiValue: dict?.value(forKey: "limit"), defaultValue: "")
let cell = self.tableView.cellForRow(at: IndexPath(row: row, section: section)) as! CustomizationDropDownTableViewCell
if self.section_quantity[section] < Int(truncating: subLimit.numberValue!) {
cell.btnDropdown.becomeFirstResponder()
}
}