Fix: Avoid null error in toc controller (#4506)

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## Because
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Currently there's a null error that can happen in the toc stimulus
controller if a lesson starts with a comment.

## This PR
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changes. -->
- Removes the possibility for this error by using optional chaining and
an early return

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Closes #4434 

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Author: JoshDevHub

app/javascript/controllers/lesson_toc_controller.js

@@ -28,7 +28,8 @@ export default class LessonTocController extends Controller {
   activeSection(entries) {
     entries.forEach((entry) => {
       const id = entry.target.getAttribute('id');
-      const tocItem = this.tocTarget.querySelector(`li a[href="#${id}"]`).parentElement;
+      const tocItem = this.tocTarget.querySelector(`li a[href="#${id}"]`)?.parentElement;
+      if (!tocItem) return;
 
       if (entry.intersectionRatio > 0) {
         tocItem.classList.add('toc-item-active');