I'm trying to convert recordRTC webm file to mp4 using ffmpeg.js library in nextjs project, during covert showing this issue, anyone can help me.

 const stopRecording = async () => {
    if (recordRef.current) {
      let blob = {}
      recordRef.current.stopRecording(function (url) {
         blob = recordRef.current.getBlob()
      });
      const webmFile = new File([blob], 'video.webm', { type: 'video/webm' })
      convertBobToMpeg(webmFile)
    }
  }


  const convertBobToMpeg = async (webmFile) => {
   

    await ffmpeg.load();
    await ffmpeg.FS('writeFile', 'video.webm', await fetchFile(webmFile))
    ffmpeg.run('-i', 'video.webm', 'demo.mp4');
    

    const data = await ffmpeg.FS('readFile', 'demo.mp4');
    setVideo_obj(URL.createObjectURL(new Blob([data.buffer], { type: 'video/mp4' })))
  }