Note that if your FormArray contains other FormGroup controls (which contain other instances of FormControl), you'll need to do this to access the controls inside each FormGroup:

        <div *ngFor="let item of myFormArray.controls; let i=index">
            <div formGroupName="{{i}}">
                <input formControlName="myFormGroupSubControl1" />
                <input formControlName="myFormGroupSubControl2" />

A simple example with FormArray @ stackblitz (gist'd below)

app.component.ts

import { Component,OnInit } from '@angular/core';
import { FormGroup, FormControl, FormArray, FormBuilder, Validators } from '@angular/forms';

@Component({
  selector: 'my-app',
  templateUrl: './app.component.html',
  styleUrls: [ './app.component.css' ]
})
export class AppComponent implements OnInit {
  fg: FormGroup;
  constructor(private fb: FormBuilder){}
  ngOnInit() {
  this.fg = this.fb.group({
    address: this.fb.group({
      street: ['', Validators.required],
    }),
    aliases: this.fb.array([])
  });
  const fa = (this.fg.get('aliases')as FormArray);
  this.addNewAlias();
  }
  addNewAlias(){
    const fa = (this.fg.get('aliases')as FormArray);
    fa.push(this.fb.group({
      name: ['', Validators.required]
    }));
  }
  deleteAlias(i:number){
    const fa = (this.fg.get('aliases')as FormArray);
    fa.removeAt(i);
    if(fa.length===0) this.addNewAlias();
  }
}

app.component.html

<form [formGroup]="fg" class="spaced">
  <h3>Nested in Group:</h3>
  <div formGroupName="address" class="spaced">
    <label>
      <input type="text" formControlName="street">
      valid: {{fg.get('address').get('street')?.valid}}
    </label>
  </div>
  <h3>Nested in Array:</h3>
  <div formArrayName="aliases" *ngFor="let alias of fg.get('aliases').controls; let i = index;" class="border">
    <div [formGroupName]="i">
      <label>
        Alias {{i+1}}:
        <input formControlName="name" placeholder="Item name">valid: {{alias.get('name')?.valid}}
      </label>
      <button type="button" (click)="deleteAlias(i)">X</button>
    </div>
  </div>
  <button type="button" (click)="addNewAlias()">add</button>
</form>
<div>form valid: {{fg?.valid}}</div>

You must use the interpolation.

Instead of:

    <div
      class="form-group"
      *ngFor="let hobbyControl of getControls(); let i = index">
      <input type="text" class="form-control" formControlName="i">
    </div>

You must use the interpolation with "formControlName":

    <div
      class="form-group"
      *ngFor="let hobbyControl of getControls(); let i = index">
      <input type="text" class="form-control" [formControlName]="i">
    </div>

If you're a using a recent version of Angular, make sure to do the operations in the TypeScript file, instead of in the Web page, as shown below in the file "app.component.ts":

  getControls() {
    return (<FormArray>this.signupForm.get('hobbies')).controls;
  }

I also encountered this error, and I solved this problem by initiating the variables of class(elements in this.fb.array([])).

Code Snippet

    mobileNumbers: this.fb.array([this.fb.group(new MobileNumber('IN'))]),

Where class MobileNumber is used.

export class MobileNumber{
    public country_code: string;
    public mobile_number: string;
    constructor($cc){
        this.country_code = COUNTRY_CODES[$cc];
    }
}

To

export class MobileNumber{
    public country_code = '';
    public mobile_number = '';
    constructor($cc){
        this.country_code = COUNTRY_CODES[$cc];
    }
}

-> Adding FormArray in component file

 //TO USE FormArray (set form1:any;)
 form1:any;

  hobbies: new FormArray([
     new FormControl()
  ])

-> To Add/Delete controls to/from hobbies

 addHobbies(){
   this.form1.get('hobbies').push(new FormControl());
 }

removeHobbies(index:number){
   this.form1.get('hobbies').removeAt(index);
}

-> Template File <!------- [FormArray] ADD HOBBIES------->

<div formArrayName="hobbies">
   <label for="hobbies">Hobbies</label>
   <div *ngFor="let hobby of form1.get('hobbies').controls;index as i">
    <input type='text' id="hobbies" class="form-control" placeholder="hobbies" [formControlName]="i">
    <button type="button" (click)="removeHobbies(i)">remove</button>
  </div>
  <button type="button (click)="addHobbies()">Add</button> 
 </div>