I was wondering if you could switch:
add_action( 'init', array( 'Widget_Data', 'init' ) );
add_action( 'widgets_init', array( 'Widget_Data', 'init' ), 20 );
The plugin is working great, but I use the WooSidebars plugin in themes and the 'init' action fires before the custom WooSidebars are registered on 'widgets_init'. I am including a modified version of this plugin in my themes to work with WooSidebars, but thought others may be registering sidebars on 'widgets_init' as well and losing this great and very useful functionality of your plugin.