Post editor blank page after adding Defer Parsing Javascript

tldr;

I’m using this code to defer parsing js

function defer_parsing_of_js ( $url ) {
  if ( FALSE === strpos( $url, '.js' ) ) return $url;
  if ( strpos( $url, 'jquery.js' ) ) return $url;
  if ( strpos( $url, 'jquery.min.js' ) ) return $url;
    return "$url' defer ";
}
add_filter( 'clean_url', 'defer_parsing_of_js', 11, 1 );

site working properly, but when I go to post editor, it just completely blank, white blank. Try to remove that defer, and the post editor works as before. Why is this happening ? Is there something wrong with the code ? Or I’m missing something ? FYI, I’m not a coder, I got that code from some site (forgot what’s the site). Thank you.