Widgets: Add missing actions to widgets block editor

Adds the `'sidebar_admin-setup'`, `'sidebar_admin_page'`, and
`current_theme_supports( 'widgets' )` check to the widgets block editor so that
the block editor screen is more compatible with the classic screen.

Fixes #53288.
Props isabel_brison.


git-svn-id: https://develop.svn.wordpress.org/trunk@51061 602fd350-edb4-49c9-b593-d223f7449a82

Author: noisysocks

src/wp-admin/widgets-form-blocks.php

@@ -55,6 +55,9 @@
 /** This action is documented in wp-admin/edit-form-blocks.php */
 do_action( 'enqueue_block_editor_assets' );
 
+/** This action is documented in wp-admin/widgets-form.php */
+do_action( 'sidebar_admin_setup' );
+
 require_once ABSPATH . 'wp-admin/admin-header.php';
 
 /** This action is documented in wp-admin/widgets-form.php */
@@ -64,4 +67,7 @@
 <div id="widgets-editor" class="blocks-widgets-container"></div> 
 
 <?php
+/** This action is documented in wp-admin/widgets-form.php */
+do_action( 'sidebar_admin_page' );
+
 require_once ABSPATH . 'wp-admin/admin-footer.php';

src/wp-admin/widgets-form.php

@@ -72,10 +72,6 @@
 	'<p>' . __( '<a href="https://wordpress.org/support/">Support</a>' ) . '</p>'
 );
 
-if ( ! current_theme_supports( 'widgets' ) ) {
-	wp_die( __( 'The theme you are currently using isn&#8217;t widget-aware, meaning that it has no sidebars that you are able to change. For information on making your theme widget-aware, please <a href="https://developer.wordpress.org/themes/functionality/widgets/">follow these instructions</a>.' ) );
-}
-
 // These are the widgets grouped by sidebar.
 $sidebars_widgets = wp_get_sidebars_widgets();
 

src/wp-admin/widgets.php

@@ -20,6 +20,10 @@
 	);
 }
 
+if ( ! current_theme_supports( 'widgets' ) ) {
+	wp_die( __( 'The theme you are currently using isn&#8217;t widget-aware, meaning that it has no sidebars that you are able to change. For information on making your theme widget-aware, please <a href="https://developer.wordpress.org/themes/functionality/widgets/">follow these instructions</a>.' ) );
+}
+
 $title       = __( 'Widgets' );
 $parent_file = 'themes.php';