Gtk4中Gtk.Video()组件无法播放视频的问题求助

Gtk4发布已有一段时间，它新增了**Gtk.Video()**组件，无需依赖Gstreamer即可在Gtk窗口中显示视频。这是一个非常简洁的类，仅有4个子类、10个方法和2个属性。

但我始终找不到任何示例（甚至C语言的也没有），于是编写了如下代码：

#!/usr/bin/env python3

import sys
import gi
gi.require_version('Gtk', '4.0')
gi.require_version('Adw', '1')
from gi.repository import Gtk, Adw, Gio

class MainWindow(Gtk.ApplicationWindow):
    def __init__(self, *args, **kwargs):
        super().__init__(title="Mini Player", *args, **kwargs)
        player = Gtk.Video.new()
        player.set_autoplay(True)
        print('file: ', player.props.file) # => None
        file_to_play = Gio.File.new_for_path('/my/valid/video/file.mp4')
        player.set_file(file_to_play) # => this is supposed to start the playing
        self.set_child(player)
        print('file: ', player.props.file) # => file: __gi__.GLocalFile object
        print('autoplay: ', player.props.autoplay) # => True
        # self.show() # I tried this too, it does nothing
        
class MyApp(Adw.Application):
    def __init__(self, **kwargs):
        super().__init__(**kwargs)
        self.connect('activate', self.on_activate)
        self.connect('open', self.on_open)
        self.set_flags(Gio.ApplicationFlags.HANDLES_OPEN)  # Need to tell GApplication we can handle this
        self.win = None
        
    def on_activate(self, app):
        self.win = MainWindow(application=app)
        self.win.present()
    
    # This is to avoid the error "Your application claims to support opening files but does not implement g_application_open() and has no handlers connected to the 'open' signal." while I learn how to pass this file to the Mainwindow class
    def on_open(self, app, files, n_files, hint):
        self.on_activate(app)
        for file in files:
            print("File to open: " + file.get_path())  
            
app = MyApp(application_id="com.example.GtkApplication")
app.run(sys.argv)

窗口能正常显示，左下角有播放按钮，控制台无警告或错误信息，但视频无法播放，点击播放按钮也无反应，窗口始终为黑屏。

请问我是否遗漏了什么关键步骤？

内容的提问来源于stack exchange，提问作者yPhil