title: Form group async validator function not getting called
body: <p>I need to implement async validation dependent on multiple form fields, so I placed the validation on <code>FormGroup</code>. But the validation function is not getting called. Surely am missing something.</p>
  
  <pre><code>heroForm: FormGroup;
  
    ngOnInit(): void {
      this.heroForm = new FormGroup({
        'name': new FormControl(this.hero.name, [
          Validators.required,
          Validators.minLength(4),
          forbiddenNameValidator(/bob/i)
        ]),
        'alterEgo': new FormControl(this.hero.alterEgo, {        
          updateOn: 'blur'
        }),
        'power': new FormControl(this.hero.power, Validators.required)
      }, null,this.validate.bind(this));
    }
  
    validate(
      ctrl: FormGroup
    ): Promise&lt;ValidationErrors | null&gt; | Observable&lt;ValidationErrors | null&gt; {
      console.log(ctrl)
      return this.heroesService.isAlterEgoTaken(ctrl.controls.alterEgo.value).pipe(
        map(isTaken =&gt; { 
          console.log(isTaken);
          return (isTaken ? { uniqueAlterEgo: true } : null)
          }),
        catchError(() =&gt; null)
      );
    }
  </code></pre>
  
  <p>Created a demo here: <a href="https://stackblitz.com/edit/angular-xtqhqi" rel="nofollow noreferrer">https://stackblitz.com/edit/angular-xtqhqi</a></p>