diff --git src/wp-includes/class-wp-customize-widgets.php src/wp-includes/class-wp-customize-widgets.php
index 0b7b7ce..0a73953 100644
--- src/wp-includes/class-wp-customize-widgets.php
+++ src/wp-includes/class-wp-customize-widgets.php
@@ -99,7 +99,7 @@ final class WP_Customize_Widgets {
 		}
 
 		add_filter( 'customize_dynamic_setting_args',          array( $this, 'filter_customize_dynamic_setting_args' ), 10, 2 );
-		add_action( 'after_setup_theme',                       array( $this, 'register_settings' ) );
+		add_action( 'widgets_init',                            array( $this, 'register_settings' ), 95 );
 		add_action( 'wp_loaded',                               array( $this, 'override_sidebars_widgets_for_theme_switch' ) );
 		add_action( 'customize_controls_init',                 array( $this, 'customize_controls_init' ) );
 		add_action( 'customize_register',                      array( $this, 'schedule_customize_register' ), 1 );
@@ -376,6 +376,8 @@ final class WP_Customize_Widgets {
 	public function customize_register() {
 		global $wp_registered_widgets, $wp_registered_widget_controls, $wp_registered_sidebars;
 
+		add_filter( 'sidebars_widgets', array( $this, 'preview_sidebars_widgets' ), 1 );
+
 		$sidebars_widgets = array_merge(
 			array( 'wp_inactive_widgets' => array() ),
 			array_fill_keys( array_keys( $wp_registered_sidebars ), array() ),
@@ -509,8 +511,6 @@ final class WP_Customize_Widgets {
 				$this->manager->get_setting( $new_setting_id )->preview();
 			}
 		}
-
-		add_filter( 'sidebars_widgets', array( $this, 'preview_sidebars_widgets' ), 1 );
 	}
 
 	/**
diff --git tests/phpunit/tests/customize/widgets.php tests/phpunit/tests/customize/widgets.php
index 28392fd..f5eb464 100644
--- tests/phpunit/tests/customize/widgets.php
+++ tests/phpunit/tests/customize/widgets.php
@@ -70,6 +70,11 @@ class Tests_WP_Customize_Widgets extends WP_UnitTestCase {
 
 	function set_customized_post_data( $customized ) {
 		$_POST['customized'] = wp_slash( wp_json_encode( $customized ) );
+		if ( $this->manager ) {
+			foreach ( $customized as $id => $value ) {
+				$this->manager->set_post_value( $id, $value );
+			}
+		}
 	}
 
 	function do_customize_boot_actions() {