Login on Android using Rest Slim

I need help to make a comparison with the value returned from the function in PHP, in the application it shows this error: java.lang.NullPointerException

Code on Android

contactSelect = new JsonClassSelect().execute(new String[]{"http://meuIP:8080/meucaminho/login/" + email+"/"+senha});
    email = "";
    try {
        krueger = contactSelect.get().toString();
        if ("0".equals(krueger))
        {
            Toast.makeText(getApplicationContext(), "Usuário e/ ou senha inválidos!", Toast.LENGTH_LONG).show();
        }
        else
        {
            txtResult.setText(krueger);
        }
        //Toast.makeText(getApplicationContext(), contact.get().toString(), Toast.LENGTH_LONG).show();
        } catch (InterruptedException e) {
        // TODO Auto-generated catch block
            e.printStackTrace();
        } catch (ExecutionException e) {
        // TODO Auto-generated catch block
            e.printStackTrace();
        }
   }

PHP Code

function loginUsuario($nm_email_usuario, $cd_senha_usuario){
    $stmt=getConn()->query("SELECT * FROM tb_usuario where nm_email_usuario='$nm_email_usuario' and cd_senha_usuario='".md5($cd_senha_usuario)."'");
    $usuario=$stmt->fetchAll(PDO::FETCH_OBJ);
    if (!empty($usuario))
    {
        echo "{\"Usuario\":".json_encode($usuario)."}";
    }
    else
    {
        $usuario= "0";
        echo json_encode($usuario);'
    }
}