title: getting controls inside nested formArray
body: <p>I was how wondering how to get controls inside a nested formArray. I initialized this form</p>
  <pre><code> ngOnInit(): void {
       this.surveyForm = this.formBuilder.group({
         'surveyTitle': [null],
         'surveyDescription': new FormControl(null),
         'questionsDetail': this.formBuilder.array([
           this.formBuilder.group({
            'questionType': new FormControl('mcq'),
             'question': new FormControl(null),
             'choices': this.formBuilder.array([])
           })
         ])
       });
  </code></pre>
  <p>but when i try to access the controls of my choices formArray, I get an error. I used this code</p>
  <pre><code>get questionsDetailcontrols() {
      return (this.surveyForm.get('questionsDetail') as FormArray).controls;
    }
    get choicesControl()
    {
      return (this.questionsDetailcontrols.get('choices') as FormArray).controls;
    }
  </code></pre>
  <p>I get an error at get('choices') stating that &quot;Property 'get' does not exist on type 'AbstractControl[]'&quot;.
  Can someone show me how to access a controls inside a nester array.</p>
  <p>Thanks in Advance</p>