can not ready property 'controls' of null when using formArray

I'm trying to use a reactive form that contains an array, however I'm getting:

  

Can not read property 'controls' of null

I tried something like:

TS

  formularioRegistro: FormGroup;
  contas_mp: FormArray;

  ngOnInit() {
    this.criarFormularioDeUsuario();
  }

  criarFormularioDeUsuario() {
    this.formularioRegistro = this.fb.group({
      email: ['', Validators.compose([Validators.required, Validators.email])],
      password: ['', Validators.compose([Validators.required])],
      nome: ['', Validators.compose([Validators.required])],
      telefone: ['', Validators.compose([Validators.required])],
      possui_whatsapp: [''],
      contas_mp: this.fb.array([this.criaContaMP()])
    });
  }

  criaContaMP(): FormGroup{
    return this.fb.group({
      conta_mp: ''
    })
  }

  adicionaContaMp(): void{
    this.contas_mp = this.formularioRegistro.get('contas_mp') as FormArray;
    this.contas_mp.push(this.criaContaMP());
  }

TEMPLATE:

<div formArrayName="contas_mp" *ngFor="let contaMP of formularioRegistro.get('contas_mp').controls; let i = index;">
   <div [formGroupName]="i">
        <input formControlName="conta_mp" placeholder="Conta MP {{i+1}}">
   </div>
</div>