This error message has the ability to drive some developers completely insane. However, the solution to this error is A LOT easier than you think.
What to do
- Identify the file where the error originates from. This is stated in the actual error. In this case it is “functions.php”, not “theme-editor.php”
- Remove the white spaces before the “<?php” or “<?” tags as well as after the “?>” tag.
Believe it or not, you are done. Easy as pie.