Check that edit site function exists

- otherwise will cause fatal when FSE
  experiment is disabled

lib/widgets.php

@@ -21,7 +21,7 @@ function gutenberg_is_block_editor() {
 		(
 			$screen->is_block_editor() ||
 			'gutenberg_page_gutenberg-widgets' === $screen->id ||
-			gutenberg_is_edit_site_page( $screen->id )
+			( function_exists( 'gutenberg_is_edit_site_page' ) && gutenberg_is_edit_site_page( $screen->id ) )
 		);
 }