title: Type 'Observable<User | null>' is not assignable to type 'Observable<User>'
body: <p>So here is the authService.ts :</p>
  <pre><code> user$: Observable&lt;firebase.User&gt; 
  
   constructor(private afAuth: AngularFireAuth) { 
   this.user$ = afAuth.authState ;  //&lt;-------- here's the problem
    }```                                                          
                                       
  </code></pre>