[Sitemap_index.xml] XML Parsing Error with Yoast and the Hemingway Theme (1 pos

Hi,

I’m a big fan of the Yoast. Thanks so much.

I use it to boost my SEO. But it seems that there’s a problem with my current theme.

I tested everything plugins, theme… It appears that the source of the bug is the Theme. Do you have any idea where I should look at?

XML Parsing Error: XML or text declaration not at start of entity
    Location: https://site.com/blog/sitemap_index.xml/
    Line Number 3, Column 1:<?xml version="1.0" encoding="UTF-8"?><?xml-stylesheet type="text/xsl" href="//site.com/blog/main-sitemap.xsl"?>
    ^

Cheers,
ZNB.

https://www.ads-software.com/plugins/wordpress-seo/