Sir, recently my sitemap http://www.whatvwant.com/sitemap_index.xml generated by wordpress seo by yoast plugin giving error.

This page contains the following errors:

error on line 3 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.

I searched for the solution in yoast forum. The answer is

Check your themes functions.php file and other plugins for trailing whitespace (or opening whitespace) outside of the <?php and ?> tags.

I checked functions.php file, but I could not find whitespace. And I could not find ?> tag also.

Please let me know the solution.