title: _rawValidators of null on adding new FormGroup dynamicly
body: <p>I am trying to create a dynamic form and push some formGoups to the form, when a button is clicked.</p>
  <p>here is some (reduced) code..<br />
  creating the form</p>
  <pre><code>form: FormGroup;
  influencer: FormArray;
  collaborations: FormArray;
  
  ngOnInit(): void {
    this.form = this.formBuilder.group({
      _id: ['', Validators.required],
      influencer: this.formBuilder.array([]),
    });
  }
  </code></pre>
  <p>creating influencer form group and collaboration group</p>
  <pre><code>createInfluencer() {
    return this.formBuilder.group({
      name: ['', Validators.required],
      collaborations: this.formBuilder.array([])
    });
  }
  
  createCollaboration(){
    return this.formBuilder.group({
      name: ['', Validators.required],
      logoUrl: ['', Validators.required]
    });
  }
  </code></pre>
  <p>Button events to create the groups:</p>
  <pre><code>addInfluencer(): void {
    this.influencer = this.form.get('influencer') as FormArray;
    this.influencer.push(this.createInfluencer());
  }
  
  addCollaboration(index): void {
    const influencer = this.form.get('influencer') as FormArray;
    this.collaborations = influencer.controls[index].get('collaborations') as FormArray;
    this.collaborations.push(this.createCollaboration());
  }
  </code></pre>
  <p>Creating Influencer Group works nice, but on creating a collaboration group i`m getting this error:</p>
  <pre><code>ERROR TypeError: Cannot read property '_rawValidators' of null
  </code></pre>
  <p>And i not unterstand why this occurs. Can someone explain why the error occurs?</p>