I have logged this as a bug in our system to be corrected in the next update.
to correct in your current site:
in the webassist/file_manipulation/helperphp.php file, change:
if(file_exists($path) && is_writable($fullPath)){
unlink($fullPath);
$result = TRUE;
}
to:
if(file_exists($path) && is_file($path) && is_writable($fullPath)){
unlink($fullPath);
$result = TRUE;
}