Fixed error in `grace.shutdown` happens if use it outside of its module context

src/utils/grace.js

@@ -42,7 +42,7 @@ module.exports = {
 	
 	async shutdown(code) {
 		logInfo('Server is shutting down gracefully...');
-		await this.handleShutdown();
+		await module.exports.handleShutdown();
 		process.exit(code || 0);
 	}