HarmonyOS读取视频封缩略图问题求助

import media from '@ohos.multimedia.media'; 
import image from '@ohos.multimedia.image'; 
import { BusinessError } from '@ohos.base'; 
import picker from '@ohos.file.picker'; 
import fs from '@ohos.file.fs'; 
import photoAccessHelper from '@ohos.file.photoAccessHelper'; 
@Entry 
@Component 
struct Index{ 
  // pixelMap对象声明，用于图片显示 
  @State pixelMap: image.PixelMap | string | undefined = undefined; 
  build() { 
    Column() { 
      Button() { 
        Text('select').fontSize(20).fontWeight(FontWeight.Bold) 
      } 
      .margin({ top: 20 }) 
      .width(100) 
      .height(40) 
      .onClick(() => this.showPicker()) 
      Image(this.pixelMap).width(300).height(300).margin({ top: 20 }) 
    } 
    .width('100%') 
    .height('100%') 
  } 
  showPicker() { 
    // 创建图库选择器对象实例 
    const photoViewPicker = new photoAccessHelper.PhotoViewPicker(); 
    // 图库选项配置，类型与数量 
    let selectOptions = new photoAccessHelper.PhotoSelectOptions(); 
    selectOptions.MIMEType = photoAccessHelper.PhotoViewMIMETypes.VIDEO_TYPE; 
    selectOptions.maxSelectNumber = 1; 
    //调用select()接口拉起图库界面进行文件选择,文件选择成功后，返回PhotoSelectResult结果集 
    photoViewPicker.select(selectOptions).then((photoSelectResult: picker.PhotoSelectResult) => { 
      //用一个全局变量存储返回的uri 
      if (photoSelectResult.photoUris?.[0]) { 
        this.testFetchFrameByTime(photoSelectResult.photoUris[0]) 
      } 
    }).catch((err: BusinessError) => { 
      console.error(`Invoke photoViewPicker.select failed, code is ${err.code}, message is ${err.message}`); 
    }) 
  } 
  // 获取缩略图 
  async testFetchFrameByTime(filePath: string) { 
    // 创建AVImageGenerator对象 
    let avImageGenerator: media.AVImageGenerator = await media.createAVImageGenerator() 
    let file = fs.openSync(filePath, fs.OpenMode.READ_ONLY); 
    let avFileDescriptor: media.AVFileDescriptor = { fd: file.fd }; 
    avImageGenerator.fdSrc = avFileDescriptor; 
    // 初始化入参 
    let timeUs = 0 
    let queryOption = media.AVImageQueryOptions.AV_IMAGE_QUERY_NEXT_SYNC 
    let param: media.PixelMapParams = { 
      width : 300, 
      height : 400, 
    } 
    // 获取缩略图（promise模式） 
    this.pixelMap = await avImageGenerator.fetchFrameByTime(timeUs, queryOption, param) 
    // 释放资源（promise模式） 
    avImageGenerator.release() 
    console.info(`release success.`) 
    fs.closeSync(file); 
  } 
}