NextJs - How to generate XML file

I'm new to NextJs and trying to create an xml file. I followed below article but 'getServerSideProps' method was not triggered and I see below message in console

API resolved without sending a response for /api/sitemap.xml, this may result in stalled requests.

https://www.learnbestcoding.com/post/16/nextjs-generate-sitemap-dynamically#sitemapXml

const Sitemap = () => {};
const toUrl = (route) =>
  `<url><loc>${route.url}</loc><lastmod>${route.lastmod}</lastmod></url>`;
  
const createSitemap = (urlList) => 
  `<?xml version="1.0" encoding="UTF-8"?>
    <urlset xmlns="https://www.sitemaps.org/schemas/sitemap/0.9">
    ${urlList.map((url) => toUrl(url)).join("")}
    </urlset>`;
    
export async function getServerSideProps({ res, req }) {      
    const siteMapJson = await fetch(`https://www.exampleapi.com/getsitemap`);
    console.log("::::::::::::::::::",siteMapJson)
    const urlList = await siteMapJson.json();
    const sitemap = createSitemap(urlList);
    res.setHeader("Content-Type", "text/xml");
    res.write(sitemap);
    res.end();
    return { props: { results : {urlList}}}
};
export default Sitemap;

Can anyone help me.