使用 http 服务的角度自定义验证 - 无法读取未定义的属性

运行代码时出现此错误：无法读取未定义的属性“getClientEmails”。如果我从 ngOnInit 调用“clientService”，而不是从“shouldBeUnique”方法内部调用它，它会起作用

export class ClientFormComponent {

  emails: any = {};

  form = new FormGroup({
    email: new FormControl('',
      null,
      this.shouldBeUnique),

    password: new FormControl('', Validators.required),
    confirm: new FormControl('', Validators.required),
    firstname: new FormControl('', Validators.required),
    lastname: new FormControl('', Validators.required),
    address: new FormControl('', Validators.required),
    phone: new FormControl('', Validators.required),
    medical: new FormControl('', Validators.required)
  });

  constructor(public clientService: ClientService) { }


  shouldBeUnique(control: AbstractControl): Promise<ValidationErrors | null> {

    this.clientService.getClientEmails().subscribe(email => {
      this.emails = email;
      console.log(this.emails);
    });

    return new Promise((resolve, reject) => {

      if (control.value === '')
        resolve({ shouldBeUnique: true });
      else
        resolve(null);
    });
  }
}