PHP警告：main（）：节点不再存在

I sometimes get this error message in my logfile.

01.03.2015 18:43:04 [client 87.177.90.0] PHP Warning: main(): Node no longer exists in *****.php on line 108

Line 108 is:

$genre_id = $category_id['im']['id'];

I tried several ways to check if the node is available, but it don´t get it done. Can anyone help me on this?

$Kategorien = simplexml_load_file('http://itunes.apple.com/de/rss/topsongs/limit=10/explicit=true/xml');

foreach ($Kategorien->entry as $item) {
    $id =  $item->id;
    $title = $item->title;
    $preview = $item->link[1]["href"];
    $namespaces = $item->getNameSpaces(true);
    $im = $item->children($namespaces['im']);
    $track_title =  $im->name;
    $track_artist = $im->artist;
    $track_amount = $im->price->attributes()->amount;
    $track_currency = $im->price->attributes()->currency;
    $release_date =  $im->releaseDate;
    $image =  $im->image[2];
    $entry_id['im'] = $item->id->attributes('im', TRUE);
    $track_id =  $entry_id['im']['id'];
    $category_id['im'] = $item->category->attributes('im', TRUE);
    $genre_id = $category_id['im']['id'];
    $genre_cat =  $item->category->attributes()->term;

}

I tried this way, but without any success. Where is my mistake?

foreach ($Kategorien->entry as $item) {
    $id =  $item->id;
    $title = $item->title;
    $preview = $item->link[1]["href"];
    $namespaces = $item->getNameSpaces(true);
    $im = $item->children($namespaces['im']);
    $track_title =  $im->name;
    $track_artist = $im->artist;
    $track_amount = $im->price->attributes()->amount;
    $track_currency = $im->price->attributes()->currency;
    $release_date =  $im->releaseDate;
    $image =  $im->image[2];
    $entry_id['im'] = $item->id->attributes('im', TRUE);
    $track_id =  $entry_id['im']['id'];
    $category_id['im'] = $item->category->attributes('im', TRUE);

if (isset($category_id['im']['id'])) {

    $genre_id = $category_id['im']['id'];
    $genre_cat =  $item->category->attributes()->term;
}
}