Pull Request resolves #48713 .

• I read the Generative AI policy and my contribution is either not created with the help of AI or is compatible with the policy and GNU/GPL 2 or later.

Summary of Changes

When the component router cannot be created (for example a backend-only component that registers a RouterFactory but has no site router class), Route::_() catches the RuntimeException and returns null. The System - SEF plugin did not check for this and called $app->redirect(null, 301), which redirects to the site root and raises Warning: Uninitialized string offset 0 in AbstractWebApplication::redirect().

This PR skips the redirect when Route::_() returns null, both in enforceSEF() (strict routing) and in the tainted URL branch of onAfterRoute(). The request is then handled as it would be with strict routing disabled.

Testing Instructions

1. Make sure System - SEF is enabled with "Strict Routing" = Yes (default) and Search Engine Friendly URLs is on in Global Configuration. Set Error Reporting to Maximum.
2. Install a backend-only component that registers a RouterFactory without a site router, e.g. Akeeba Backup or Regular Labs ReReplacer, or the minimal test component attached here (com_routertest.zip).
3. Open in the frontend: index.php?option=com_routertest&view=json&json=x (or option=com_akeebabackup / option=com_rereplacer).
4. Open a core non-SEF URL as a control, e.g. index.php?option=com_users&view=login.
5. Apply the PR and repeat steps 3 and 4.

Actual result BEFORE applying this Pull Request

Step 3: 301 redirect to the homepage, and Warning: Uninitialized string offset 0 in .../AbstractWebApplication.php is raised.
Step 4: 301 redirect to the SEF URL index.php/component/users/login.

Expected result AFTER applying this Pull Request

Step 3: no redirect and no warning; the request is handled by the component (the attached test component has no site part, so Joomla shows a 404, same as with Strict Routing = No).
Step 4: unchanged, 301 redirect to the SEF URL.

Link to documentations

Please select:

• Documentation link for guide.joomla.org:

• No documentation changes for guide.joomla.org needed

• Pull Request link for manual.joomla.org:

• No documentation changes for manual.joomla.org needed

com_routertest.zip