Skip init function if stylesheet not found

Instead of throwing an error about a member not being defined or accessed on NULL.

assets/scripts/app.js

@@ -10,6 +10,7 @@ const app = new modular({
 window.onload = (event) => {
     const $style = document.getElementById('main-css');
 
+    if ($style) {
         if ($style.isLoaded) {
             init();
         } else {
@@ -17,6 +18,9 @@ window.onload = (event) => {
                 init();
             });
         }
+    } else {
+        console.warn('The "main-css" stylesheet not found');
+    }
 };
 
 function init() {