Theme JSON Resolver: Update cache check to also check that the object…

… is an instance of the Gutenberg version (#42756)

* Theme JSON Resolver: Update cache check to also check that the object is an instance of the Gutenberg version

* Add comment for when the resolver is backported to core

lib/experimental/class-wp-theme-json-resolver-gutenberg.php

@@ -33,7 +33,8 @@ public static function get_theme_data( $deprecated = array(), $settings = array(
 			_deprecated_argument( __METHOD__, '5.9' );
 		}
 
-		if ( null === static::$theme ) {
+		// When backporting to core, remove the instanceof Gutenberg class check, as it is only required for the Gutenberg plugin.
+		if ( null === static::$theme || ! static::$theme instanceof WP_Theme_JSON_Gutenberg ) {
 			$theme_json_data = static::read_json_file( static::get_file_path_from_theme( 'theme.json' ) );
 			$theme_json_data = static::translate( $theme_json_data, wp_get_theme()->get( 'TextDomain' ) );
 			$theme_json_data = gutenberg_add_registered_webfonts_to_theme_json( $theme_json_data );