Site Editor: Fix PHP warning pre-loading page REST requests.

Fix a PHP warning when opening a page directly in the site editor, eg by navigating to the URL `/wp-admin/site-editor.php?p=%2Fpage&postId=200`, caused by an undefined variable name.

Props abcd95, joemcgill, narenin, soyebsalar, websiteredev, wildworks.
Fixes #63122.


git-svn-id: https://develop.svn.wordpress.org/trunk@60061 602fd350-edb4-49c9-b593-d223f7449a82

Author: peterwilsoncc

src/wp-admin/site-editor.php

@@ -239,7 +239,7 @@ static function ( $classes ) {
 			$preload_paths[] = add_query_arg(
 				'slug',
 				// @see https://github.com/WordPress/gutenberg/blob/e093fefd041eb6cc4a4e7f67b92ab54fd75c8858/packages/core-data/src/private-selectors.ts#L244-L254
-				empty( $block_editor_context->post->post_name ) ? 'page' : 'page-' . $post->post_name,
+				empty( $block_editor_context->post->post_name ) ? 'page' : 'page-' . $block_editor_context->post->post_name,
 				'/wp/v2/templates/lookup'
 			);
 		}