fix(search): add null-check in dropdown hide handler (#3983)

# Description of Changes

- **What was changed**  
- Added a null-check on `dropdownMenu` in the document click handler to
prevent errors when the menu element is not present.

- **Why the change was made**  
- To guard against potential runtime errors if `dropdownMenu` is `null`.

```js
search.js:126 Uncaught TypeError: Cannot read properties of null (reading 'classList')
    at HTMLDocument.<anonymous> (search.js:126:60)
(anonymous)	@	search.js:126
```

---

## Checklist

### General

- [x] I have read the [Contribution
Guidelines](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/CONTRIBUTING.md)
- [x] I have read the [Stirling-PDF Developer
Guide](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/devGuide/DeveloperGuide.md)
(if applicable)
- [ ] I have read the [How to add new languages to
Stirling-PDF](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/devGuide/HowToAddNewLanguage.md)
(if applicable)
- [x] I have performed a self-review of my own code
- [x] My changes generate no new warnings

### Documentation

- [ ] I have updated relevant docs on [Stirling-PDF's doc
repo](https://github.com/Stirling-Tools/Stirling-Tools.github.io/blob/main/docs/)
(if functionality has heavily changed)
- [ ] I have read the section [Add New Translation
Tags](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/devGuide/HowToAddNewLanguage.md#add-new-translation-tags)
(for new translation tags only)

### UI Changes (if applicable)

- [ ] Screenshots or videos demonstrating the UI changes are attached
(e.g., as comments or direct attachments in the PR)

### Testing (if applicable)

- [ ] I have tested my changes locally. Refer to the [Testing
Guide](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/devGuide/DeveloperGuide.md#6-testing)
for more details.

---------

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

app/core/src/main/resources/static/js/search.js

@@ -82,13 +82,20 @@ document.querySelector("#navbarSearchInput").addEventListener("input", function
   resultsBox.style.width = window.navItemMaxWidth + "px";
 });
 
-
+document.addEventListener('DOMContentLoaded', function () {
   const searchDropdown = document.getElementById('searchDropdown');
   const searchInput = document.getElementById('navbarSearchInput');
 
-// Check if elements exist before proceeding
-if (searchDropdown && searchInput) {
+  // Check if elements are missing and skip initialization if necessary
+  if (!searchDropdown || !searchInput) {
+    console.warn('Search dropdown or input not found. Skipping initialization.');
+    return;
+  }
   const dropdownMenu = searchDropdown.querySelector('.dropdown-menu');
+  if (!dropdownMenu) {
+    console.warn('Dropdown menu not found within the search dropdown. Skipping initialization.');
+    return;
+  }
 
   // Create a single dropdown instance
   const dropdownInstance = new bootstrap.Dropdown(searchDropdown);
@@ -133,4 +140,4 @@ if (searchDropdown && searchInput) {
     e.stopPropagation();
   });
 
-}
+});