要解决这个问题，你需要确保在播放视频之前先把视频加载到AVPlayer中。下面是一个示例代码：

import UIKit
import AVKit
import AVFoundation

class ViewController: UIViewController, UIImagePickerControllerDelegate, UINavigationControllerDelegate {

    var player: AVPlayer?
    var playerViewController: AVPlayerViewController?
    
    override func viewDidLoad() {
        super.viewDidLoad()
    }
    
    func playVideo() {
        guard let videoURL = // 获取UIImagePickerController返回的视频URL else {
            return
        }
        
        player = AVPlayer(url: videoURL)
        
        playerViewController = AVPlayerViewController()
        playerViewController?.player = player
        
        present(playerViewController!, animated: true) {
            self.player?.play()
        }
    }
    
    func openImagePicker() {
        let imagePicker = UIImagePickerController()
        imagePicker.delegate = self
        imagePicker.sourceType = .photoLibrary
        imagePicker.mediaTypes = ["public.movie"]
        
        present(imagePicker, animated: true, completion: nil)
    }
    
    // UIImagePickerControllerDelegate方法
    func imagePickerController(_ picker: UIImagePickerController, didFinishPickingMediaWithInfo info: [UIImagePickerController.InfoKey : Any]) {
        if let videoURL = info[.mediaURL] as? URL {
            player?.pause() // 如果正在播放其他视频，先暂停
            player = nil // 释放之前的player
            playerViewController?.dismiss(animated: true, completion: nil)
            
            playVideo()
        }
        
        picker.dismiss(animated: true, completion: nil)
    }
}

在这个示例中，创建了一个AVPlayer和一个AVPlayerViewController实例来播放视频。当用户选择视频后，imagePickerController(_:didFinishPickingMediaWithInfo:)方法会被调用，其中的代码会先暂停之前的视频播放，并释放之前的AVPlayer实例，然后再重新加载新的视频并播放。