If the pages that are giving errors have no content, you can deal with it by adding a check at the start of the function:
function image_manipulation( $content ) {
if(empty($content)) return $content;
//rest of the code
}
If the pages that are giving errors have no content, you can deal with it by adding a check at the start of the function:
function image_manipulation( $content ) {
if(empty($content)) return $content;
//rest of the code
}