Issue #3474535 by mably: fix generated html with unordered headings

Closes #3474535

assets/js/tocjs.js

@@ -368,8 +368,18 @@
         lastLi = li;
       } else if (level < currentLevel) {
         while (level < currentLevel) {
-          currentList = currentList.parentNode.parentNode;
-          currentLevel = currentList.level;
+          // Special case where the parent element is the root nav element.
+          // Can occur when the first heading is not of the highest level.
+          // See: https://www.drupal.org/project/toc_js/issues/3474535
+          if (currentList.parentNode.nodeName === 'NAV') {
+            if (level < currentList.level) {
+              currentList.level = level;
+            }
+            break;
+          } else {
+            currentList = currentList.parentNode.parentNode;
+            currentLevel = currentList.level;
+          }
         }
         currentList.appendChild(li);
         lastLi = li;