ffmpeg.FS('readFile', 'demo.mp4') error. Check if the path exists

I'm trying to convert recordRTC webm file to mp4 using ffmpeg.js library in nextjs project, during conversion showing this issue

ffmpeg.FS('readFile', 'demo.mp4') error. Check if the path exists

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'
    })))
}