Refactor extension.ts to use optional chaining for baseUrl

src/extension.ts

@@ -1,16 +1,3 @@
-// Copyright (c) Jupyter Development Team.
-// Distributed under the terms of the Modified BSD License.
-
-// Entry point for the notebook bundle containing custom model definitions.
-//
-// Setup notebook base URL
-//
-// Some static assets may be required by the custom widget javascript. The base
-// url for the notebook is not known at build time and is therefore computed
-// dynamically.
-// eslint-disable-next-line @typescript-eslint/no-non-null-assertion
-(window as any).__webpack_public_path__ =
-  document.querySelector('body')!.getAttribute('data-base-url') +
-  'nbextensions/ipychess';
-
+const baseUrl = document.querySelector('body')?.getAttribute('data-base-url');
+(window as any).__webpack_public_path__ = `${baseUrl}\nnbextensions/ipychess`;
 export * from './index';