function handle_register_user()
{
    if (!isset($_POST['_mynonce']) || !wp_verify_nonce($_POST['_mynonce'], 'register-user')) {
        echo "error";
        die();
    }

    $email = $_POST['email'];
    $password = $_POST['password'];
    $username = $_POST['username'];

    $user_id = wp_create_user( $username, $password, $email );
    $url = plugin_dir_url( __FILE__ ) . 'template/user_dashboard.php';

    if (!is_int($user_id)) {
        echo "error";
        return;
    }
    wp_redirect($url.'?id='.$user_id);

    exit;

}

<?php

$id = $_GET['id'];
global $wpdb;
var_dump($wpdb);exit;//null

$query = "SELECT * FROM wp_users WHERE ID = $id";

$result = $wpdb->get_results($query);
var_dump($result);