title: Angular safe navigation operator in property binding
body: <p>Why is the safe navigation operator in Angular(4) not working with property binding? The following html code produces an error (as it will try to <code>GET</code> <a href="http://localhost:4200/null" rel="nofollow noreferrer">http://localhost:4200/null</a>)</p>
  
  <pre><code>    &lt;img [src]="(image | async)?.url" /&gt;
  </code></pre>
  
  <p>But then when I change it to the following expression, everything works fine. </p>
  
  <pre><code>    &lt;img src="{{(image | async)?.url}}" /&gt;
  </code></pre>