pdo fatal error - call to member function prepare() on null in C:\xampp\htdocs\folder1\folder2\

class Database{
private $host = DB_HOST;
private $user = DB_USER;
private $pass = DB_PASS;
private $dbname = DB_NAME;

private $dbh;
private $error;

private $stmt;


public function __construct()
{
    // Set DSN

    $dsn = 'mysql:host=' . $this->host . ';dbname=' .               $this->dbname;

    // Set options
    $options = array(
    PDO::ATTR_PERSISTENT => true, 
    PDO::ATTR_ERRMODE => PDO::ERRMODE_EXCEPTION,
    PDO::MYSQL_ATTR_INIT_COMMAND => "SET NAMES utf8",
    PDO::MYSQL_ATTR_INIT_COMMAND => "SET CHARACTER SET utf8");

    try {
    $this->dbh = new PDO($dsn, $this->user, $this->pass, $options);
    } catch (PDOException $e) {
    $this->error = $e->getMessage();
    }
}

public function query($query)
{
    $this->stmt = $this->dbh->prepare($query);

}

public function bind($param, $value, $type = null)
{
    if (is_null($type)) {
    switch (true) {
    case is_int($value):
    $type = PDO::PARAM_INT;
    break;
    case is_bool($value):
    $type = PDO::PARAM_BOOL;
    break;
    case is_null($value):
    $type = PDO::PARAM_NULL;
    break;
    default:
    $type = PDO::PARAM_STR;
            }
    }
    $this->stmt->bindValue($param, $value, $type);
}

public function execute(){
    return $this->stmt->execute();
}

public function resultset(){
    $this->execute();
    return $this->stmt->fetchAll(PDO::FETCH_ASSOC);
}

public function single(){
    $this->execute();
    return $this->stmt->fetch(PDO::FETCH_ASSOC);
}

public function rowCount(){
    return $this->stmt->rowCount();
}

public function lastInsertId(){
    return $this->dbh->lastInsertId();
}

public function beginTransaction(){
    return $this->dbh->beginTransaction();
}

public function endTransaction(){
    return $this->dbh->commit();
}

public function cancelTransaction(){
    return $this->dbh->rollBack();
}

public function debugDumpParams(){
    return $this->stmt->debugDumpParams();
}



}

// Instantiate database.
$database = new Database();

$stmt= $database->query("CREATE TABLE mytable (
ID int(11) NOT NULL AUTO_INCREMENT,
FName varchar(50) NOT NULL,
LName varchar(50) NOT NULL,
Age int(11) NOT NULL,
Gender enum('male','female') NOT NULL,
PRIMARY KEY (ID)
) ENGINE=MyISAM  DEFAULT CHARSET=latin1 AUTO_INCREMENT=11 ");
$sql = $database->prepare($stmt);


$database->query('INSERT INTO mytable (FName, LName, Age, Gender) VALUES (:fname, :lname, :age, :gender)');

$database->bind(':fname', 'John');
$database->bind(':lname', 'Smith');
$database->bind(':age', '24');
$database->bind(':gender', 'male');

$database->execute();

echo $database->lastInsertId();