woo-product-importer non-static method fix

<?php /*
    Plugin Name: Woo Product Importer
    Plugin URI: http://webpresencepartners.com/2012/09/19/a-free-simple-woocommerce-csv-importer/
    Description: Free CSV import utility for WooCommerce
    Version: 1
    Author: Daniel Grundel, Web Presence Partners
    Author URI: http://www.webpresencepartners.com
    Text Domain: woo-product-importer
    Domain Path: /languages/
*/

/*
    This file is part of Woo Product Importer.

    Woo Product Importer is Copyright 2012-2013 Web Presence Partners LLC.

    Woo Product Importer is free software: you can redistribute it and/or modify
    it under the terms of the GNU Lesser General Public License as published by
    the Free Software Foundation, either version 3 of the License, or
    (at your option) any later version.

    Woo Product Importer is distributed in the hope that it will be useful,
    but WITHOUT ANY WARRANTY; without even the implied warranty of
    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
    GNU Lesser General Public License for more details.

    You should have received a copy of the GNU Lesser General Public License
    along with Woo Product Importer.  If not, see <http://www.gnu.org/licenses/>.
*/

    class WebPres_Woo_Product_Importer {

        public function __construct() {
            add_action('init', array( &$this, 'translations' ), 1 );
            add_action('admin_menu', array( &$this, 'admin_menu') );
            add_action('wp_ajax_woo-product-importer-ajax', array( &$this, 'render_ajax_action') );
        }

        public function translations() {
            load_plugin_textdomain('woo-product-importer', false, dirname( plugin_basename( __FILE__ ) ) . '/languages/');
        }

        public function admin_menu() {
            add_management_page(
                __('Woo Product Importer', 'woo-product-importer'),
                __('Woo Product Importer', 'woo-product-importer'),
                'manage_options',
                'woo-product-importer',
                array( &$this, 'render_admin_action' )
            );
        }

        public function render_admin_action() {
            $action = isset( $_REQUEST['action'] ) ? $_REQUEST['action'] : 'upload';

            require_once( plugin_dir_path(__FILE__) . 'woo-product-importer-common.php');
            require_once( plugin_dir_path(__FILE__) . "woo-product-importer-{$action}.php");
        }

        public function render_ajax_action() {
            require_once( plugin_dir_path(__FILE__) . "woo-product-importer-ajax.php");
            die(); // this is required to return a proper result
        }
    }

    $webpres_woo_product_importer = new WebPres_Woo_Product_Importer();