[5.3] Fix warnings on frontend template options editing

[joomdonation]

Pull Request for Issue #45050.

Summary of Changes

This PR fixes warnings on frontend template options editing page. See #45050 for details issue description.

Testing Instructions

1. Use latest Joomla 5.3
2. Create a menu item to link to Display Template Options menu item type
3. Login to frontend of your site using a super user account
4. Access to above menu item

Actual result BEFORE applying this Pull Request

You see warning: Warning: SimpleXMLElement::xpath(): Unfinished literal in \libraries\src\Form\Form.php on line 1259

Expected result AFTER applying this Pull Request

No warnings anymore

Link to documentations

Please select:

• Documentation link for docs.joomla.org:

• No documentation changes for docs.joomla.org needed

• Pull Request link for manual.joomla.org:

• No documentation changes for manual.joomla.org needed

[QuyTon]

I have tested this item ✅ successfully on 3cdcb69

---

_{This comment was created with the J!Tracker Application at issues.joomla.org/tracker/joomla-cms/45053.}

[cyrez]

I have tested this item ✅ successfully on 3cdcb69

Turned on error reporting first.

Works as expected, warning removed.

---

_{This comment was created with the J!Tracker Application at issues.joomla.org/tracker/joomla-cms/45053.}

[QuyTon]

RTC

---

_{This comment was created with the J!Tracker Application at issues.joomla.org/tracker/joomla-cms/45053.}

[laoneo]

Thanks!