Error connecting to the BDD through a class

I am using a class that a user provided me with in a question that I published some time ago.

The fact is that through this class I can make the connection to the database and make queries, etc.

I was doing a connection test when when executing a query the following error appeared:

  

Notice: Undefined property: Db :: $ stmt in   C: \ xampp \ htdocs \ php \ data_conexion.php on line 94

     

Fatal error: Uncaught Error: Call to a member function bindValue () on   null in C: \ xampp \ htdocs \ php \ data_conexion.php: 94 Stack trace: # 0   C: \ xampp \ htdocs \ php \ index.php (22): Db-> bind (': user', 'gmarsi') # 1   {main} thrown in C: \ xampp \ htdocs \ php \ data_conexion.php on line 94

The code I use is this:

<?php 

require("datos_conexion.php");

$dbObj = new Db();
$pdoObj = $dbObj -> getInstance();

$user = '';
$name = 'marcos';
$user = 'gmarsi';
$data = 'SELECT User_User FROM users WHERE User_Punc > 0 ';

if($user != ''){
    $data .= " AND User_User = :user ";
    $pdoObj -> bind(':user', 'gmarsi');
} 

if($name != ''){
    $data .= " AND User_Name = :name ";
    $pdoObj -> bind(':name', $name);
} 

echo $data;
$pdoObj -> query($data);
$resultado = $pdoObj ->resultset();

foreach($resultado as $row){
    echo '<br>'.$row['User_User'];
}

?>

And the class in question is this:

<?php
class Db 
{

private $host      = 'localhost';
private $user      = 'admin';
private $dbname    = 'baseDeDatos';
private $pass      = 'contraseña';
private static $instance;

public function __construct()
{
    // DSN: cadena de conexión
    $dsn = 'mysql:host=' . $this->host . ';dbname=' . $this->dbname;

    /* Opciones
        * Muy importante, para tener una buena conexión PDO

    */    
    $options = array(
        PDO::ATTR_PERSISTENT => true, //establecer a false si no se quieren conexiones persistentes
        PDO::ATTR_EMULATE_PREPARES => false, //no cambiar nunca
        PDO::ATTR_ERRMODE => PDO::ERRMODE_EXCEPTION, //no cambiar nunca
        PDO::MYSQL_ATTR_INIT_COMMAND => "SET NAMES 'utf8'" //juego de caracteres
    );

    // Crea nueva instancia de PDO
    try{
        $this->dbh = new PDO($dsn, $this->user, $this->pass, $options);
    }
    // Captura los errores
    catch(PDOException $e){
        $this->error = $e->getMessage();
    }
}



public static function getInstance()
{
    if (self::$instance == null) 
    {
        $className = __CLASS__;
        self::$instance = new $className();
    }
    return self::$instance;
}



 /******************************************** MÉTODOS IMPRESCINDIBLES */

//Recibe las consultas
public function query($query)
{
    $this->stmt = $this->dbh->prepare($query);
}

//Ejecuta las consultas
public function execute()
{
    return $this->stmt->execute();
}

//Devuelve resultados de la consulta en PDO::FETCH_ASSOC 
//Se puede cambiar o agregar otro método con otro tipo de resultado
public function resultset()
{
    $this->execute();
    return $this->stmt->fetchAll(PDO::FETCH_ASSOC);

}

//Envía los datos por separado para proteger de la inyección SQL
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, PDO::PARAM_STR);
}

/********************************************** FIN MÉTODOS IMPRESCINDIBLES




/******************************************** MÉTODOS OPCIONALES */

//Devuelve el último registro insertado
public function lastInsertId()
{
    return $this->dbh->lastInsertId();
}

//Obtiene un registro simple
public function single()
{
    $this->execute();
    return $this->stmt->fetch(PDO::FETCH_ASSOC);
}


//Devuelve un solo valor, columna o 0
public function valor()
{
    $this->execute();
    $valor=$this->stmt->fetch(PDO::FETCH_COLUMN);
    return (empty($valor)) ? 0 : $valor;

}

//Verifica si un registro existe devolviendo true o false
public function si_existe()
{
    $this->execute();
    if ($this->stmt->fetch(PDO::FETCH_ASSOC))
    {
        return true;
    } 
    else 
    {
        return false;
    }
}
 /**********************************************/

}

?>

I leave the link to the answer where I got this class, since the user made an example of use. Here