AngularBootstrap 的 select 占位符不起作用

typescript
import { Directive, ElementRef, OnInit } from '@angular/core';
@Directive({
  selector: '[placeholderFix]'
})
export class PlaceholderFixDirective implements OnInit {
  constructor(private elementRef: ElementRef) { }
  ngOnInit() {
    const selectElement = this.elementRef.nativeElement;
    const placeholderOption = document.createElement('option');
    placeholderOption.disabled = true;
    placeholderOption.selected = true;
    placeholderOption.hidden = true;
    placeholderOption.textContent = selectElement.getAttribute('placeholder');
    selectElement.prepend(placeholderOption);
  }
}

html
<select placeholder="请选择" placeholderFix [(ngModel)]="selectedValue">
  <option *ngFor="let option of options" [value]="option.value">{{ option.label }}</option>
</select>