Fix window reference error for service workers

Author: andreadev-it

index.js

@@ -2,5 +2,5 @@ import BackgroundHandler from './src/BackgroundHandler.js';
 import BackgroundScript from './src/BackgroundScript.js';
 
 // Makes these classes globally available 
-window.BackgroundHandler = BackgroundHandler;
-window.BackgroundScript = BackgroundScript;
+globalThis.BackgroundHandler = BackgroundHandler;
+globalThis.BackgroundScript = BackgroundScript;