Set checkbox validation if checkbox unchecked

In my Angular 6 application's register form, I have a checkbox which is already checked. I need to check whether the user has unchecked the checkbox and if it is so I want to display an error message.

This is my checkbox in .html file,

 <div class="form-group">
       <div class="round">
            <input
                  type="checkbox"
                  id="checkbox33"
                  formControlName="agree">
             <label for="checkbox33"></label>
        </div>
        <small class="text-muted">Agree to our <a href="#" class="link">Terms
                  Of Use</a> and <a href="#" class="link">Privacy Policy</a>.
                </small>
        <span *ngIf="!worldWideRegisterForm.get('agree').valid" class="text-danger">You should agree!</span>

  </div>

This is my ngOnInit() in .ts file

ngOnInit() {
    this.worldWideRegisterForm = new FormGroup({
      'userName': new FormGroup({
        firstName: new FormControl(null, Validators.required),
        lastName: new FormControl(null, Validators.required)
      }),
      'email': new FormControl(null, [Validators.required, Validators.email]),
      'phone': new FormControl(null, Validators.required),
      'address': new FormGroup({
        line1: new FormControl(null, Validators.required),
        line2: new FormControl(null),
      }),
      'area': new FormGroup({
        'province': new FormControl(null, Validators.required),
        'city': new FormControl(null, Validators.required),
        'postalCode': new FormControl(null, Validators.required),
      }),
      'password': new FormControl(null, [Validators.required, Validators.minLength(6)]),
      'agree': new FormControl(true, Validators.required)
    });
  }