Non-WWW sitemap not visible

Hi,

I am using Yoast SEO Version 1.4.25 on Monkshouts. When I visit WWW based sitemap , I get all the sitemaps but when I visit the nonWWW sitemap by yoast , a blank page appears.

Is there any way can we rename the sitemap_index.xml to sitemap.xml as Yoast website says that the file was renamed because they wanted to avoid conflicts with other sitemap plugins.

If someone had changed the permalink structure from NONwww to WWW, and had both the sites in google webmaster tools, should we submit sitemaps for both the versions of website if both the versions are pointing to same location on server? According to http://sitemaps.blogspot.in/2005/12/www-vs-non-www-versions-of-site.html , we should not submit sitemaps for both of them.

I guess if these questions can be answered, many people will get help as I searched a lot for all these queries, but updated answers are not available.

https://wordpress.org/plugins/wordpress-seo/