从DOM获取Element后，PHP DOM附加了子元素

Question

Currently my XML looks like

<?xml version="1.0" ?>
<hero-attributes>
    <hero-weaknesses>
        <hero>
            IO
        </hero>
    </hero-weaknesses>
</hero-attributes>

I am trying to add

<hero>Wisp</hero>

after

<hero>IO</hero>

I currently have

<?php
$dir = opendir("../../heroes/");
while ($listing = readdir($dir))
{
    if (substr($listing, 0, 1) == ".")
        continue;

    $dom = new DOMDocument();
    $url = '../../heroes/' . $listing;
    @$dom->load($url);

    $element = $dom->getElementsByTagName('hero-weaknesses')->item(0);
    if(is_null($element))
        die('No element found');

    $hero = $dom->createElement('hero', "IO");
    $hero = $element->appendChild($hero);

    $heroName = $dom->createTextNode('Wisp');
    $heroName = $hero->appendChild($heroName);
}
?>

the error I am getting (When i remove the null check)

Fatal error: Call to a member function appendChild() on a non-object in C:\GitHub\--\public_html\Scripts\GetHeros.php on line 15

I have no idea why $element is NULL and what the problem is