XML error on sitemap.xml

I look for empty lines inside every PHP files but I didn't find any but at last, I found the solution the error was due to a function which I declare in my functions.php file, the function was register_sidebar(); but I was not using it in my theme anywhere. When I removed this function in my functions.php file then sitemap.xml of my website working correctly

This is probably due to empty lines inside your custom php files. Try checking every php file you edited for empty lines after the ?> closing tag: the tag should be the last character of any file.