puppeteer 爬取网易云网站问题

转自 @huyiyang 的代码： 获取网易云的歌词和评论

const fs = require('fs');
const puppeteer = require('puppeteer');

(async () => {
  const browser = await (puppeteer.launch({ executablePath: '/Users/huqiyang/Documents/project/z/chromium/Chromium.app/Contents/MacOS/Chromium', headless: false }));
  const page = await browser.newPage();
  // 进入页面
  await page.goto('https://music.163.com/#');

  // 点击搜索框拟人输入 鬼才会想起
  const musicName = '鬼才会想';
  await page.type('.txt.j-flag', musicName, {delay: 0});

  // 回车
  await page.keyboard.press('Enter');

  // 获取歌曲列表的 iframe
  await page.waitFor(2000);
  let iframe = await page.frames().find(f => f.name() === 'contentFrame');
  const SONG_LS_SELECTOR = await iframe.$('.srchsongst');

  // 获取歌曲 鬼才会想起 的地址
  const selectedSongHref = await iframe.evaluate(e => {
    const songList = Array.from(e.childNodes);
    const idx = songList.findIndex(v => v.childNodes[1].innerText.replace(/\s/g, '') === '鬼才会想起');
    return songList[idx].childNodes[1].firstChild.firstChild.firstChild.href;
  }, SONG_LS_SELECTOR);

  // 进入歌曲页面
  await page.goto(selectedSongHref);

  // 获取歌曲页面嵌套的 iframe
  await page.waitFor(2000);
  iframe = await page.frames().find(f => f.name() === 'contentFrame');

  // 点击 展开按钮
  const unfoldButton = await iframe.$('#flag_ctrl');
  await unfoldButton.click();

  // 获取歌词
  const LYRIC_SELECTOR = await iframe.$('#lyric-content');
  const lyricCtn = await iframe.evaluate(e => {
    return e.innerText;
  }, LYRIC_SELECTOR);

  console.log(lyricCtn);

  // 截图
  await page.screenshot({
    path: '歌曲.png',
    fullPage: true,
  });

  // 写入文件
  let writerStream = fs.createWriteStream('歌词.txt');
  writerStream.write(lyricCtn, 'UTF8');
  writerStream.end();

  // 获取评论数量
  const commentCount = await iframe.$eval('.sub.s-fc3', e => e.innerText);
  console.log(commentCount);

  // 获取评论
  const commentList = await iframe.$$eval('.itm', elements => {
    const ctn = elements.map(v => {
      return v.innerText.replace(/\s/g, '');
    });
    return ctn;
  });
  console.log(commentList);
})();

在18-21行，运行到：

  // 获取歌曲列表的 iframe
  await page.waitFor(2000);
  let iframe = await page.frames().find(f => f.name() === 'contentFrame');
  const SONG_LS_SELECTOR = await iframe.$('.srchsongst');
  console.log('SONG_LS_SELECTOR') // null

输出 SONG_LS_SELECTOR 为 null， 导致往后运行不下去，请问是什么原因？