HarmonyOS鸿蒙Next中asset.getThumbnail获取的缩略图怎么看不到值？

参考下面的Demo，pixelMap不一定是空的，可以在Image组件中显示出来：

import { common } from '@kit.AbilityKit';
import { photoAccessHelper } from '@kit.MediaLibraryKit';
import { dataSharePredicates } from '@kit.ArkData';
import { image } from '@kit.ImageKit';

@Entry
@Component
struct PhotoPage {
  @State imagePath: string = ''
  @State pixelMap:image.PixelMap  | undefined = undefined
  private context = this as common.UIAbilityContext;
  private uri:string = '';

  build() {
    Row() {
      Column() {
        Button("选择图片")
          .onClick(() => {
            this.selectPhoto()
          })
        Image(this.pixelMap).width(300).height(400)
      }.width('100%')
    }.height('100%')
  }

  selectPhoto() {
    let PhotoSelectOptions = new photoAccessHelper.PhotoSelectOptions();
    PhotoSelectOptions.MIMEType = photoAccessHelper.PhotoViewMIMETypes.IMAGE_TYPE;// 过滤选择媒体文件类型为IMAGE
    PhotoSelectOptions.maxSelectNumber = 1;// 选择媒体文件的最大数目
    let photoPicker = new photoAccessHelper.PhotoViewPicker();

    photoPicker.select(PhotoSelectOptions).then((photoSelectResult: photoAccessHelper.PhotoSelectResult) => {
      console.info('PhotoViewPicker.select successfully, PhotoSelectResult uri: ' + JSON.stringify(photoSelectResult));
      this.uri = photoSelectResult.photoUris[0]
      this.uriGetAssets()
    })
  }

  async uriGetAssets() {
    try {
      let phAccessHelper = photoAccessHelper.getPhotoAccessHelper(this.context);
      let predicates: dataSharePredicates.DataSharePredicates = new dataSharePredicates.DataSharePredicates();
      // 配置查询条件，使用PhotoViewPicker选择图片返回的uri进行查询
      predicates.equalTo('uri', this.uri);
      let fetchOption: photoAccessHelper.FetchOptions = {
        fetchColumns: [],
        predicates: predicates
      };
      let fetchResult: photoAccessHelper.FetchResult<photoAccessHelper.PhotoAsset> = await phAccessHelper.getAssets(fetchOption);
      // 得到uri对应的PhotoAsset对象，读取文件的部分信息
      const asset: photoAccessHelper.PhotoAsset = await fetchResult.getFirstObject();
      console.info('PhotoViewPicker asset displayName: ', asset.displayName);
      console.info('PhotoViewPicker asset uri: ', asset.uri);
      console.info('PhotoViewPicker asset photoType: ', asset.photoType);
      // 获取缩略图
      asset.getThumbnail((err, pixelMap) => {
        if (err == undefined) {
          this.pixelMap = pixelMap
          console.info('PhotoViewPicker getThumbnail successful ' + pixelMap);
        } else {
          console.error('PhotoViewPicker getThumbnail fail', err);
        }
      });
    } catch (error){
      console.error('uriGetAssets failed with err: ' + JSON.stringify(error));
    }
  }
}