Manually adding sitemap to github pages

I connected my GitHub pages to a custom domain.
after that i generated a sitemap.xml file and placed it on the root folder (where the index.html is)
but when I try to access my sitemap.
ie, domain.com/sitemap.xml it’s just showing a blank white screen.

so what is the correct way to add sitemap
and also how to do the same with robots.txt