Index: src/wp-includes/class-wp-customize-widgets.php
===================================================================
--- src/wp-includes/class-wp-customize-widgets.php	(revision 37298)
+++ src/wp-includes/class-wp-customize-widgets.php	(working copy)
@@ -1064,7 +1064,7 @@
 	 * @return array
 	 */
 	public function preview_sidebars_widgets( $sidebars_widgets ) {
-		$sidebars_widgets = get_option( 'sidebars_widgets' );
+		$sidebars_widgets = get_option( 'sidebars_widgets', array() );
 
 		unset( $sidebars_widgets['array_version'] );
 		return $sidebars_widgets;