Trying to get property of non-object in

Estou com esse erro quando tento usar PHP 7.1 ( no PHP 5.6 não dar erro )

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PHP Notice: Trying to get property of non-object in /home/themes/default/main/rsslib.php on line 18

rsslib.php

<?php

$RSS_Content = array();

function RSS_Tags($item, $type = 1, $channel_name="", $image="")

{

	$y = array();
	$tnl = $item->getElementsByTagName("title");
	$tnl = $tnl->item(0);
	$title = $tnl->firstChild->textContent;

	$tnl = $item->getElementsByTagName("link");
	$tnl = $tnl->item(0);
	$link = $tnl->firstChild->textContent;

	$tnl = $item->getElementsByTagName("pubDate");
	$tnl = $tnl->item(0);
	$date = $tnl->firstChild->textContent;

	$y["channel"] = $channel_name;
	$y["title"] = $title;
	$y["link"] = $link;
	$y["date"] = $date;
	$y["type"] = $type;
	$y["image"] = $image;

	return $y;
}

function RSS_Channel( $channel, $key )

{

	global $RSS_Content;

	$items = $channel->getElementsByTagName("item");

	$y = RSS_Tags($channel, 0);
	$channel_name = $y['title'];

	foreach($items as $item)
	{
 $y = RSS_Tags($item, 1, $channel_name, $key);

 array_push($RSS_Content, $y);
	}
}

function RSS_Retrieve( $urls = array() )

{

	global $RSS_Content;
	$RSS_Content = array();

	foreach ( $urls as $key => $url )
	{
 $doc_{$key} = new DOMDocument();
 $doc_{$key}->load( $url );
 $channels_{$key} = $doc_{$key}->getElementsByTagName("channel");

 foreach( $channels_{$key} as $channel )
 {
 RSS_Channel( $channel, $key );
 }
	}

}

function RSS_RetrieveLinks($url)

{

	global $RSS_Content;

	$doc = new DOMDocument();
	$doc->load($url);

	$channels = $doc->getElementsByTagName("channel");

	$RSS_Content = array();

	foreach($channels as $channel)
	{
 $items = $channel->getElementsByTagName("item");
 foreach($items as $item)
 {
 $y = RSS_Tags($item, 1);
 array_push($RSS_Content, $y);
 }
	}

}

function RSS_Links($url, $size = 15)

{

	global $RSS_Content;

	$page = "<ul>";

	RSS_RetrieveLinks($url);
	if($size > 0)
 $recents = array_slice($RSS_Content, 0, $size + 1);

	foreach($recents as $article)
	{
 $type = $article["type"];
 if($type == 0) continue;
 $title = $article["title"];
 $link = $article["link"];
 $page .= "<li><a href=\"$link\">$title</a></li>\n";
	}

	$page .="</ul>\n";

	return $page;
}

function RSS_Display( $urls=array(), $size = 15, $site = 0 )

{

	global $RSS_Content;

	$site = 0;
	$site = (intval($site) == 0) ? 1 : 0;

	RSS_Retrieve( $urls );
	RSS_Sort();

	if($size > 0)
	{
 $recents = array_slice($RSS_Content, 0, $size);
	}

	return $recents;

}

function RSS_Sort()

{

	global $RSS_Content;

	for( $i =0; $i < count( $RSS_Content ); $i++ )
	{
 $array = array();
 for( $j =$i+1; $j < count( $RSS_Content ) - $i; $j++ )
 {
 if( strtotime($RSS_Content[$i]['date']) < strtotime($RSS_Content[$j]['date']) )
 {
 $temp = array(
 'channel' =>	$RSS_Content[$i]['channel'],
 'title' =>	$RSS_Content[$i]['title'],
 'link' =>	$RSS_Content[$i]['link'],
 'date' =>	$RSS_Content[$i]['date'],
 //'description'	=>	$RSS_Content[$i]['description'],
 'image' =>	$RSS_Content[$i]['image'],
 'type' =>	$RSS_Content[$i]['type'],
 );

 $temp2 = array(
 'channel' =>	$RSS_Content[$j]['channel'],
 'title' =>	$RSS_Content[$j]['title'],
 'link' =>	$RSS_Content[$j]['link'],
 'date' =>	$RSS_Content[$j]['date'],
 //'description'	=>	$RSS_Content[$j]['description'],
 'type' =>	$RSS_Content[$j]['type'],
 'image' =>	$RSS_Content[$j]['image'],
 );

 $RSS_Content[$i] = $temp2;
 $RSS_Content[$j] = $temp;

 }
 }
	}
}
?>