Angular Firebase, erro: "Unable to read property & # 39; displayName & # 39; from indefinite"

meu app.component.html:

e o manuscript:

import {component} from & # 39; angular / core & # 39 ;;
import {AngularFireAuth} from & # 39; @ angular / fire / auth & # 39;
import * as a fire base from "base of fire";

@Component({
selector: "app-root",
templateUrl: & # 39; ./ app.component.html & # 39 ;,
styleUrls: ['./app.component.css']
})
AppComponent export class {
user: firebase.User;

constructor (private afAuth: AngularFireAuth) {
afAuth.authState.subscribe (user => this.user = user);
}

Connect - Out () {
this.afAuth.auth.signOut ();
}
}