Adapt JS for modular usage

assets/js/script.js

@@ -50,8 +50,8 @@ for (let i = 0; i < testimonialsItem.length; i++) {
 }
 
 // add click event to modal close button
-modalCloseBtn.addEventListener("click", testimonialsModalFunc);
-overlay.addEventListener("click", testimonialsModalFunc);
+if (modalCloseBtn) modalCloseBtn.addEventListener("click", testimonialsModalFunc);
+if (overlay) overlay.addEventListener("click", testimonialsModalFunc);