<?php
namespace Concrete\Controller;

class Feed extends \Concrete\Core\Controller\Controller
{
    public function get($identifier)
    {
        $feed = \Concrete\Core\Page\Feed::getByHandle($identifier);
        if (is_object($feed)) {
            //header('Content-Type: text/xml');
            $xml = $feed->getOutput($this->request);
            print $xml;
        }
        exit;
    }
}
<?php
namespace Concrete\Controller;

class Feed extends \Concrete\Core\Controller\Controller
{
    public function get($identifier=null, $defaultValue=null)
    {
        $feed = \Concrete\Core\Page\Feed::getByHandle($identifier);
        if (is_object($feed)) {
            //header('Content-Type: text/xml');
            $xml = $feed->getOutput($this->request);
            print $xml;
        }
        exit;
    }
}

Add default parameter values to method
Add default parameter values to   method

The \Concrete\Core\Controller\Controller::get method has a signature of get($identifier=null, $defaultValue=null). This signature was causing issues on PHP7.