delete_widget
Action Hook: Fires immediately after a widget has been marked for deletion.
Source: wp-admin/widgets.php:185
sidebar_admin_setup
Action Hook: Fires early before the Widgets administration screen loads, after scripts are enqueued.
Source: wp-admin/widgets.php:47
widgets_admin_page
Action Hook: Fires before the Widgets administration page content loads.
Source: wp-admin/widgets.php:424
sidebar_admin_page
Action Hook: Fires after the available widgets and sidebars have loaded, before the admin footer.
Source: wp-admin/widgets.php:567