您不能在 prepareForSegue 中访问 DetailViewController 的 IBOutlets，因为它们是尚未连接.

You cannot access IBOutlets of DetailViewController in prepareForSegue because they are not connected yet.

正如评论中提到的，在 DetailViewController 中创建一个 String 属性，在 prepareForSegue 中设置它并设置 text DetailViewController 的 viewDidLoad 或 viewWillAppear 中标签的属性.

As mentioned in the comments create a String property in DetailViewController, set it in prepareForSegue and set the text property of the label in viewDidLoad or viewWillAppear of DetailViewController.