Error viewing sitemapindex.xml

Hi I am getting a error message when viewing the sitemapindex.xml which is similar to the https://wordpress.org/support/topic/xml-error-when-trying-to-access-sitemapindexxml?replies=7

I tried disabling all other plugins and still could get it working. Any suggestions? The error I get is

This page contains the following errors:

error on line 5 at column 6: XML declaration allowed only at the start of the document
Below is a rendering of the page up to the first error.

Thanks for your help.

https://wordpress.org/plugins/bwp-google-xml-sitemaps/