PHP OOP Login/Register System

i started to build oop login/register system by watching tutorial on youtube (phpacademy), i got to one point and the code does not work. i checked it a few time and still nothing. DB is created and working great and i have one user in db(users)-alex. If anyone can see what i did wrong, pls help me :)

i hope i explained my problem right, the error i get is : "Fatal error: Call to a member function count() on a non-object in C:\xampp\htdocs\oopLogin\index.php on line 7"

DB.php

<?php

class DB {
    private static $_instance = null;
    private $_pdo,
            $_query, 
            $_error = false,
            $_results,
            $_count = 0;

    private function __construct() {
        try {
            $this->_pdo = new PDO('mysql:host=' . Config::get('mysql/host') . ';dbname=' . Config::get('mysql/db'), Config::get('mysql/username'), Config::get('mysql/password'));          
        } catch(PDOException $e) {
            die($e->getMessage());
        }
    }

    public static function getInstance() {
        if(!isset(self::$_instance)) {
            self::$_instance = new DB();
        }
        return self::$_instance;
    }

    public function query($sql, $params = array()) {
        $this->_error = false;
        if($this->_query = $this->_pdo->prepare($sql)) {
            $x = 1;
            if(count($params)) {
                foreach($params as $param) {
                    $this->_query->bindValue($x, $param);
                    $x++;
                }
            }

            if($this->_query->execute()) {
                $this->_results = $this->_query->fetchAll(PDO::FETCH_OBJ);
                $this->_count = $this->_query->rowCount();
            }
            else {
                $this->_error = true;
            }
        }

        return $this;
    }

    public function action($action, $table, $where = array()) {
        if(count($where) === 3){
            $operators = array('=', '>', '<', '>=', '<=');

            $field    = $where[0];
            $operator = $where[1];
            $value    = $where[2];

            if(in_array($operator, $operators)) {
                $sql = "($action) FROM ($table) WHERE ($field) ($operator) ?";

                if(!$this->query($sql, array($value))->error()) {
                    return $this;
                }
            }
        }
        return false;
    }

    public function get($table, $where) {
        return $this->action('SELECT *', $table, $where);
    }

    public function delete($table, $where) {
        return $this->action('DELETE', $table, $where);
    }

    public function results() {
        return $this->_results;
    }

    public function error() {
        return $this->_error;
    }

    public function count() {
        return $this->_count;
    }

}

index.php

<?php

require_once 'core/init.php';

$user = DB::getInstance()->get('users', array('username', '=', 'alex'));

if(!$user->count()) {
    echo 'No user';
} else {
    foreach($user->results() as $user) {
        echo $user->username, '<br>';
    }
}