ng-bootstrap和Angular 6:无法解析NgbModalRef的所有参数：(?，?)

import { Component, OnInit } from '@angular/core';
import { NgbModal, NgbModalRef } from '@ng-bootstrap/ng-bootstrap';

@Component({
  selector: 'app-example',
  template: `
    <button (click)="openModal()">打开模态框</button>
  `
})
export class ExampleComponent implements OnInit {
  modalRef: NgbModalRef;

  constructor(private modalService: NgbModal) {}

  ngOnInit() {}

  openModal() {
    this.modalRef = this.modalService.open(ModalContentComponent, {
      // 配置对象，可选
      size: 'lg',
      backdrop: 'static',
      keyboard: false
    });

    // 可以通过modalRef进行模态框的操作
    // 例如，可以订阅模态框的关闭事件
    this.modalRef.closed.subscribe(result => {
      console.log('模态框已关闭', result);
    });
  }
}

@Component({
  selector: 'app-modal-content',
  template: `
    <div class="modal-header">
      <h4 class="modal-title">模态框标题</h4>
      <button type="button" class="close" aria-label="Close" (click)="modalRef.dismiss()">
        <span aria-hidden="true">&times;</span>
      </button>
    </div>
    <div class="modal-body">
      <p>模态框内容</p>
    </div>
    <div class="modal-footer">
      <button type="button" class="btn btn-secondary" (click)="modalRef.close('关闭按钮')">关闭</button>
    </div>
  `
})
export class ModalContentComponent {
  constructor(public modalRef: NgbModalRef) {}
}