我遇到了同样的问题并找到了解决方法，虽然它不是***的。

I got the same issue and found a workaround, although it's not optimal.

您可以从文档中看到 [virtualScroll] 期待数组。当您的 this.ads 返回可观察时。这就是为什么你得到无法读取null 的属性'length'。

You see from the documentation [virtualScroll]expect an Array. While your this.ads return an Observable. That's why you got Cannot read property 'length' of null.

解决方法：首先订阅它然后显示在查看（没有异步管道），完成后也要记住unsubsribe。

Workaround: Subscribe to it first then display on view (without async pipe), also keep in mind to unsubsribe when done.

ads: any[] = [];
constructor(public adProvider: AdProvider) {
    this.adProvider.getActiveAds().valueChanges().subscribe(results => this.ads = results);  
  }