PHP Slim Rest Api

Решил написать rest api для своего приложения. Выкидывает ошибку:

Type: Error
Message: Call to a member function prepare() on null
File: C:\wamp64\www\usatu\include\DbOperation.php
Line: 15

Как я понял prepare возвращает Null, т.е проблемы с базой данной?

<?php
use \Psr\Http\Message\ServerRequestInterface as Request;
use \Psr\Http\Message\ResponseInterface as Response;
//including the required files
require_once '../include/DbOperation.php';
require '.././libs/Slim/vendor/autoload.php';

$app = new \Slim\App([
    'settings'=>[
        'displayErrorDetails'=>true
    ]
]);
/*
 * URL: http://localhost/usatu/v1/allGroups
 */
$app->get('/allGroups', function (Request $request, Response $response){
    $db = new DbOperation();
    $groups = $db->getAllStudents();
    $response_data = array();
    $response_data['error'] = false;
    $response_data['groupsStudents'] = $groups;
    $response->write(json_encode($response_data));
    return $response
        ->withHeader('Content-type', 'application/json')
        ->withStatus(200);
});
$app->run(true);

Класс операций bd

class DbOperation
{
    private $con;
    function __construct()
    {
        require_once dirname(__FILE__) . '/DbConnect.php';
        $db = new DbConnect();
        $this->con = $db->connect();
    }
    //Method to fetch all students from database
    public function getAllStudents(){
        $stmt = $this->con->prepare("SELECT * FROM class");
        $stmt->execute();
        $students = $stmt->get_result();
        $stmt->close();
        return $students;
    }

Класс DbConnect

class DbConnect
{
    private $con;
    function __construct()
    {
    }
    function connect()
    {
        include_once dirname(__FILE__) . '/Constants.php';
        $this->con = new mysqli(DB_HOST, DB_USERNAME, DB_PASSWORD, DB_NAME);
        if (mysqli_connect_errno()) {
            echo "Failed to connect to MySQL: " . mysqli_connect_error();
            return $this->con;
        }
    }
}

Полный Trace

#0 C:\wamp64\www\usatu\v1\index.php(22): DbOperation->getAllStudents()
#1 [internal function]: Closure->{closure}(Object(Slim\Http\Request), Object(Slim\Http\Response), Array)
#2 C:\wamp64\www\usatu\libs\Slim\vendor\slim\slim\Slim\Handlers\Strategies\RequestResponse.php(41): call_user_func(Object(Closure), Object(Slim\Http\Request), Object(Slim\Http\Response), Array)
#3 C:\wamp64\www\usatu\libs\Slim\vendor\slim\slim\Slim\Route.php(356): Slim\Handlers\Strategies\RequestResponse->__invoke(Object(Closure), Object(Slim\Http\Request), Object(Slim\Http\Response), Array)
#4 C:\wamp64\www\usatu\libs\Slim\vendor\slim\slim\Slim\MiddlewareAwareTrait.php(117): Slim\Route->__invoke(Object(Slim\Http\Request), Object(Slim\Http\Response))
#5 C:\wamp64\www\usatu\libs\Slim\vendor\slim\slim\Slim\Route.php(334): Slim\Route->callMiddlewareStack(Object(Slim\Http\Request), Object(Slim\Http\Response))
#6 C:\wamp64\www\usatu\libs\Slim\vendor\slim\slim\Slim\App.php(516): Slim\Route->run(Object(Slim\Http\Request), Object(Slim\Http\Response))
#7 C:\wamp64\www\usatu\libs\Slim\vendor\slim\slim\Slim\MiddlewareAwareTrait.php(117): Slim\App->__invoke(Object(Slim\Http\Request), Object(Slim\Http\Response))
#8 C:\wamp64\www\usatu\libs\Slim\vendor\slim\slim\Slim\App.php(407): Slim\App->callMiddlewareStack(Object(Slim\Http\Request), Object(Slim\Http\Response))
#9 C:\wamp64\www\usatu\libs\Slim\vendor\slim\slim\Slim\App.php(315): Slim\App->process(Object(Slim\Http\Request), Object(Slim\Http\Response))
#10 C:\wamp64\www\usatu\v1\index.php(33): Slim\App->run()
#11 {main}

Update:

$app->get('/allGroups', function (Request $request, Response $response) {
    $db = new DbConnect();
    try {
        $db = $db->connect();
        $groups = $db->query("SELECT * FROM class")->fetchAll(PDO::FETCH_OBJ);
        return $response
            ->withStatus(200)
            ->withHeader("Content-Type", 'application/json')
            ->withJson($groups);
    } catch (PDOException $e) {
        return $response->withJson(
            array(
                "error" => array(
                    "text" => $e->getMessage(),
                    "code" => $e->getCode()
                )
            )
        );
    }
    $db = null;
});
class DbConnect
{
    private $dbhost = "localhost";
    private $dbuser = "root";
    private $dbpass = "";
    private $dbname= "schedule_usatu";
    public function connect()
    {
        $mysql_connection = "mysql:host=$this->dbhost;dbname=$this->dbname;charset=utf-8";

        $connection = new PDO($mysql_connection, $this->dbuser, $this->dbpass);
        $connection->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);
        return $connection;
    }
}