XML declaration allowed only at the start of the document

When I viewed any RSS (XML) page on my WordPress site, the page didn’t render correctly and instead displayed an error stating:

XML declaration allowed only at the start of the document

Upon viewing the page source I could see that the first line in the document was in fact blank line, and the second line contained the usual XML declaration:

<?xml version="1.0" encoding="UTF-8"?>

But what was causing it?

It turned out that the problem was with my custom theme.  In Functions.php in my child theme folder I had whitespace before and after the <?php and ?> respectively.  Once I deleted this whitespace and re-uploaded Functions.php to my web server, the blank first line was gone and all of my RSS pages rendered successfully!