XML Sitemap

One of my clients is using the Zerif Pro theme, and the XML sitemap isn’t populating correctly. According to Yoast, it’s an issue with the theme creating extra space at the beginning of the XML declaration. The site itself says it’s a parsing error on line 4, column 6. Yoast said I would need to reach out to the theme dev to fix. Please advise.
Thanks,
David