title: Angular 4: Property "push" and "controls" does not exist on type "AbstractControl"
body: <p>I was implemented the code from this link <a href="http://plnkr.co/edit/yV94ZjypwBgHAlb0RLK2?p=preview" rel="noreferrer">http://plnkr.co/edit/yV94ZjypwBgHAlb0RLK2?p=preview</a> but getting push and controls error.</p>
  
  <p>Here is what i did and don't know what is wrong with it.</p>
  
  <pre><code>import { Component } from '@angular/core';
  import { ViewController,Platform } from 'ionic-angular';
  import { FormBuilder,FormGroup,Validators,FormControl,FormArray } from '@angular/forms';
  
  @Component({
    selector: 'filter-vendor',
    templateUrl: 'filter-vendor.html'
  })
  
  export class FilterVendorPage {
  
  
  
  
    questions = [{id:1,text:'Question 1', answers:[{id:1},{id:2}]},{id:2,text:'Question 2', answers:[{id:11},{id:22}]}]
    surveyForm:FormGroup;
  
  
  
    constructor(
      private viewCtrl: ViewController, 
      private formBuilder:FormBuilder
       ){
  
  
        this.surveyForm=this.formBuilder.group({
          question:formBuilder.array([])
        })
  
        for(var i=0;i&lt;this.questions.length;i++){
            let question=formBuilder.group({
              question_id:[this.questions[i].id,Validators.required],
              answer_id:formBuilder.array([])
            });
            this.surveyForm.controls['questions'].push(question);
        }
  }
  
  
     onChange(id, isChecked, index) {
  
      const answers = &lt;FormArray&gt;this.surveyForm.controls.questions.controls[index].controls.answer_ids
  
      if(isChecked) {
            answers.push(new FormControl(id))
          } else {
            let idx = answers.controls.findIndex(x =&gt; x.value == id)
            answers.removeAt(idx)
      }
    }
  
  }
  </code></pre>
  
  <p>Please help me to resolve this issue.Lot's of thanks </p>