Fix JavaScript null error on home and splash layouts (#4939)

assets/js/_main.js

@@ -123,8 +123,11 @@ $(document).ready(function () {
   });
 
   // Add anchors for headings
-  document
-    .querySelector(".page__content")
+  (function () {
+    var pageContentElement = document.querySelector(".page__content");
+    if (!pageContentElement) return;
+
+    pageContentElement
       .querySelectorAll("h1, h2, h3, h4, h5, h6")
       .forEach(function (element) {
         var id = element.getAttribute("id");
@@ -138,6 +141,7 @@ $(document).ready(function () {
           element.appendChild(anchor);
         }
       });
+  })();
 
   // Add copy button for <pre> blocks
   var copyText = function (text) {