title: @ViewChild(CdkPortalOutlet) returns undefined in AfterViewInit
body: <p>Trying to query a <code>ng-template</code> with <code>CdkPortalOutlet</code> is always unsuccessful, and I cant understand why?</p>
  
  <pre><code>&lt;ng-template CdkPortalOutlet&gt;&lt;/ng-template&gt;
  
  @ViewChild(CdkPortalOutlet) test: CdkPortalOutlet;
  </code></pre>
  
  <p><a href="https://stackblitz.com/edit/cdk-overlays-68dss7" rel="nofollow noreferrer">stackblitz</a></p>