Index: src/wp-includes/class-wp-customize-manager.php
===================================================================
--- src/wp-includes/class-wp-customize-manager.php	(revision 49243)
+++ src/wp-includes/class-wp-customize-manager.php	(working copy)
@@ -492,7 +492,25 @@
 
 		return '_default_wp_die_handler';
 	}
+	/**
+	 * Redirects a URL to the 404 page if changeset UUID does not exists
+	 *
+	 * @since 5.6.0
+	 *
+	 * @param bool     $bypass Pass-through of the pre_handle_404 filter value.
+	 * @param WP_Query $query  The WP_Query object.
+	 * @return bool Bypass value.
+	 */
+	public function customize_changeset_preview_redirect( $bypass, $query ) {
+		// If a theme/plugin has already utilized the pre_handle_404 function, return without action to avoid conflicts.
+		if ( $bypass ) {
+			return $bypass;
+		}
 
+		$query->set_404();
+		status_header( 404 );
+		return $bypass;
+	}
 	/**
 	 * Start preview and customize theme.
 	 *
@@ -521,7 +539,11 @@
 
 		// If a changeset was provided is invalid.
 		if ( isset( $this->_changeset_uuid ) && false !== $this->_changeset_uuid && ! wp_is_uuid( $this->_changeset_uuid ) ) {
+			if ( ! current_user_can( 'customize' ) && ! $this->changeset_post_id() ) { //Adding pre_handle_404 for unauthenticated Invalid changeset UUID
+				 add_filter( 'pre_handle_404', array( $this, 'customize_changeset_preview_redirect' ), 10, 2 );
+			} else {
 			$this->wp_die( -1, __( 'Invalid changeset UUID' ) );
+		    }
 		}
 
 		/*
@@ -548,7 +570,7 @@
 		 * then send unauthenticated code to prompt re-auth.
 		 */
 		if ( ! current_user_can( 'customize' ) && ! $this->changeset_post_id() ) {
-			$this->wp_die( $this->messenger_channel ? 0 : -1, __( 'Non-existent changeset UUID.' ) );
+			add_filter( 'pre_handle_404', array( $this, 'customize_changeset_preview_redirect' ), 10, 2 );
 		}
 
 		if ( ! headers_sent() ) {