Default widgets assignment – isn’t working

I solved the issue using a fallback with PHP conditionals: in my sidebar.php I coded like the following:

<?php if( is_active_sidebar('my_sidebar') ) { ?>

        <div id="secondary" class="widget-area">
            <ul class="xoxo">
                <?php dynamic_sidebar('my_sidebar'); ?>
            </ul>
        </div><!-- #secondary .widget-area -->

<?php } else { ?>

        <div id="secondary" class="widget-area">
            <ul class="xoxo">

                <?php // Default Widget : Archives ?>
                <li id="default-widget-1" class="widget-container default-widgets">
                    <h3 class="widget-title"><?php _e( 'Archives', 'your-theme' ); ?></h3>
                    <ul>
                        <?php wp_get_archives( array( 'type' => 'monthly' ) ); ?>
                    </ul>
                </li> <!-- #default-widget-1 -->

                <?php // Default Widget : Pages ?>
                <li id="default-widget-2" class="widget-container default-widgets">
                    <h3 class="widget-title"><?php _e( 'Pages', 'your-theme' ); ?></h3>
                    <ul>
                        <?php wp_list_pages('sort_column=menu_order&title_li='); ?>
                    </ul>
                </li> <!-- #default-widget-2 -->

                <?php // Default Widget : Categories ?>
                <li id="default-widget-3" class="widget-container default-widgets">
                    <h3 class="widget-title"><?php _e( 'Categories', 'your-theme' ); ?></h3>                            
                    <ul>
                        <?php wp_list_categories( 'sort_column=menu_order&title_li=' ); ?>
                    </ul>
                </li> <!-- #default-widget-3 -->

                <?php // Default Widget : Meta ?>
                <li id="default-widget-4" class="widget-container default-widgets">
                    <h3 class="widget-title"><?php _e( 'Meta', 'your-theme' ); ?></h3>                            
                    <ul>
                        <?php wp_register(); ?>
                        <li><?php wp_loginout(); ?></li>
                        <?php wp_meta(); ?>
                    </ul>
                </li> <!-- #default-widget-4 -->

            </ul> <!-- .xoxo -->

        </div><!-- #secondary .widget-area -->

<?php } //endif( is_active_sidebar ?>