PHP mysqli致命错误：在null上调用成员函数query（）

Im getting this error and i dont know how to fix.

'Fatal error: Call to a member function query() on null in C:\xampp\htdocs\cms\lib\db.php on line 46'

Code:

class db {

public $mysqli;

function Connect() {

         $server = 'localhost';
        $dbusername = 'root';
        $database = 'cms';
        $dbpassword = 'lopaka';
        $this->mysqli = new mysqli($server, $dbusername, $dbpassword, $database);

        /* connectie bekijken */
        if (mysqli_connect_errno()) {
            printf("Connectie mislukt: %s
", mysqli_connect_error());
            exit();
        }
    }


function GetGallery() {

    $query = "SELECT * FROM menu";
    $action = $this->mysqli->query($query);

    while ($row = $action->fetch_assoc()) {
        $gallery .= '<a href="'.$siteurl.''.$row['url'].'" title="'.$row['alt'].'" data-gallery="" ><img src="'.$siteurl.''.$row['url'].'" width="75px" height="75px"></a>';    
    }

    return $gallery;
} 

}

Thanks !!!

Edit - works now, but now i have some new errors:

Notice: Undefined index: url in C:\xampp\htdocs\cms\lib\db.php on line 42

Notice: Undefined index: alt in C:\xampp\htdocs\cms\lib\db.php on line 42

Notice: Undefined index: url in C:\xampp\htdocs\cms\lib\db.php on line 42

     while ($row = mysqli_fetch_assoc($action)) {
            $gallery .= '<a href="'.$siteurl.''.$row['url'].'" title="'.$row['alt'].'" data-gallery="" ><img src="'.$siteurl.''.$row['url'].'" width="75px" height="75px"></a>';    
        }
 return $gallery;

You should add __construct() method to your class... add like this...

public function __construct()
{           
    $this->Connect();
}

Here the Connect() will initialized and then only mysqli query() method will work.