ItemsSource绑定到集合保持为空

I have a class YouTubeVideo which contains some variables, which I would like to show on the UI. Therefore I created a custom control which shows all the data of the class. There are more than one instances of the class, so I would like to use an ItemsControl to show them on the screen like in a StackPanel. I defined a ViewModel for the ItemsControl and I'm adding the custom usercontrols from my window code behind to the ViewModel, however the ItemsControl stays empty. which contains some