角度4:AbstractControl类型上不存在属性“push”和“controls”

我是从这个链接实现代码的 http://plnkr.co/edit/yV94ZjypwBgHAlb0RLK2?p=preview

以下是我所做的,我不知道这有什么问题。

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<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 = <FormArray>this.surveyForm.controls.questions.controls[index].controls.answer_ids

    if(isChecked) {
          answers.push(new FormControl(id))
        } else {
          let idx = answers.controls.findIndex(x => x.value == id)
          answers.removeAt(idx)
    }
  }

}

请帮我解决这个问题。非常感谢