ngBootstrap for Angular 4 总是需要反应形式的 DatePicker

ngBootstrap for Angular 4 DatePicker in a Reactive Form is Always Required

我使用反应形式添加了 ngbDatePicker:

<input type="text"
        id="business_incorp_date"
        class="form-control"
        formControlName="business_incorp_date"
        ngbDatepicker
        #incorporatedDatePicker="ngbDatepicker"
        (click)="incorporatedDatePicker.toggle()"
        readonly>

表单模型为:

this.form = this.formBuilder.group({
  id: [
    0, [Validators.required]
  ],

  // ... removed for brevity

  business_type: [
    '', [Validators.required]
  ],
  business_incorp_date: [
    '', [FormValidators.date] // <-- NOT REQUIRED BY MODEL
  ],
  business_desc: [
    '', [Validators.required]
  ]

  // ...
});

但是,如果该字段为空,它仍然是 ngbDateFormat:

的 ngInvalid
{ "ngbDate": { "invalid": { "year": null, "month": null, "day": null } } }

有人知道如何让 DatePicker 字段允许空字符串吗?

@pkozlowski.opensource 通过 github issue 提供的答案指出必须将模型设置为 null 以避免对可选字段进行验证。