如何在没有初始值设定项的formGroup中设置子formGroup？

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

@Component({
  selector: 'app-dynamic-form',
  templateUrl: './dynamic-form.component.html',
  styleUrls: ['./dynamic-form.component.css']
})
export class DynamicFormComponent implements OnInit {
  form: FormGroup;

  constructor(private fb: FormBuilder) {}

  ngOnInit(): void {
    this.form = this.fb.group({
      // 父 FormGroup
      parentControl: ['', Validators.required],
      childGroup: this.fb.group({
        // 子 FormGroup
        childControl1: ['', Validators.required],
        childControl2: ['', Validators.minLength(3)]
      })
    });
  }

  onSubmit(): void {
    if (this.form.valid) {
      console.log(this.form.value);
    } else {
      console.log('Form is invalid');
    }
  }
}

<form [formGroup]="form" (ngSubmit)="onSubmit()">
  <div>
    <label for="parentControl">Parent Control:</label>
    <input id="parentControl" formControlName="parentControl">
  </div>
  <div formGroupName="childGroup">
    <div>
      <label for="childControl1">Child Control 1:</label>
      <input id="childControl1" formControlName="childControl1">
    </div>
    <div>
      <label for="childControl2">Child Control 2:</label>
      <input id="childControl2" formControlName="childControl2">
    </div>
  </div>
  <button type="submit">Submit</button>
</form>