title: Error using catch() with a .on() in firebase
body: <p>I am retrieving data in firebase using the following code:</p>
  
  <pre><code>  this.managerList = this.afDB.database.ref('users').orderByChild('managedby').equalTo(uID);
        this.managerList.on('value', (snapshot) =&gt; {
          this.managerIdArray = Object.keys(snapshot.val());
          this.managerNameArray = snapshot.val();
         });
  </code></pre>
  
  <p>Whenever a null value is returned, I get an error :
  Error: Uncaught (in promise): TypeError: Cannot read property............ of undefined.</p>
  
  <p>When I try to add a catch() to the above, it says cannot use catch() or then(). How do I use a catch() to take error.</p>