Sitemap Problem

I’m getting this error when using the Spasalon theme.

This page contains the following errors:
error on line 1 at column 9: XML declaration allowed only at the start of the document
Below is a rendering of the page up to the first error.

Looking at my page source I find this:

			<?xml version="1.0" encoding="UTF-8"?><?xml-stylesheet type="text/xsl" href="//xxxxxxxxxxx.nz/main-sitemap.xsl"?>
<sitemapindex xmlns="https://www.sitemaps.org/schemas/sitemap/0.9">

Note the space before the ?xml. I think this is what is causing the problem.

If I change to the Twentytwenty theme the error disappears.

I have checked my functions.php file and there is no blank space at either the top or bottom of the file, which I believe is something that could cause the error.