How to use the angular-oauth2-oidc.NullValidationHandler function in angular-oauth2-oidc

// URL of the SPA to redirect the user after silent refresh
    this.oauthService.silentRefreshRedirectUri =
      window.location.origin + '/silent-refresh.html';

    // The SPA's id. The SPA is registerd with this id at the auth-server
    this.oauthService.clientId = 'spa-demo';

    // set the scope for the permissions the client should request
    // The first three are defined by OIDC. The 4th is a usecase-specific one
    this.oauthService.scope = 'openid profile email voucher';

    // Url of the Identity Provider
    this.oauthService.issuer =
      'https://steyer-identity-server.azurewebsites.net/identity';

    this.oauthService.tokenValidationHandler = new NullValidationHandler();

    this.oauthService.events.subscribe(e => {
      // tslint:disable-next-line:no-console
      console.debug('oauth/oidc event', e);
    });

    // Load Discovery Document and then try to login the user
    this.oauthService.loadDiscoveryDocument().then(doc => {
      this.oauthService.tryLogin();
    });

    this.oauthService.events
      .pipe(filter(e => e.type === 'token_expires'))
      .subscribe(e => {
        // tslint:disable-next-line:no-console
        console.debug('received token_expires event', e);

private configureWithoutDiscovery() {
    this.oauthService.configure(noDiscoveryAuthConfig);
    this.oauthService.tokenValidationHandler = new NullValidationHandler();
    this.oauthService.tryLogin();
  }

constructor(
    private oauthService: OAuthService,
    private router: Router
  ) {
    this.oauthService.configure(authConfig);
    this.oauthService.tokenValidationHandler = new NullValidationHandler();

    this.oauthService.events
      .subscribe(_ => {
        this.isAuthenticatedSubject$.next(this.oauthService.hasValidAccessToken());
      });

    this.oauthService.setupAutomaticSilentRefresh();
  }