Undefined array key "#prefix" in ManagedFile::uploadAjaxCallback() when file element is nested in media reference widget

Problem/Motivation

In ManagedFile::uploadAjaxCallback() (line 198 of core/modules/file/src/Element/ManagedFile.php), the code uses the .= concatenation operator to append rendered status messages to $form['#prefix']:

$form['#prefix'] .= $renderer->renderRoot($status_messages);

This assumes that #prefix already exists on the form element retrieved via NestedArray::getValue(). While #prefix is normally set during processManagedFile() (line 372), there are cases where the AJAX form rebuild resolves to an element that hasn't had its #prefix initialized — specifically when a file upload widget is deeply nested within a media reference field on a node or block edit form.

This results in a PHP warning:

Warning: Undefined array key "#prefix" in Drupal\file\Element\ManagedFile::uploadAjaxCallback()

The warning can break the AJAX response, causing the file upload to fail intermittently. The issue is timing/form-cache dependent, making it non-deterministic.

Steps to reproduce

1. Enable a contrib module that extends the image widget's file handling (e.g. drupal/svg_image).
2. Create a content type with a media reference field that accepts image media (including SVG).
3. Edit a node of that content type.
4. Use the inline media widget to upload an SVG file directly from the node edit form.
5. Observe the PHP warning in logs and intermittent upload failures.
6. Note: uploading the same file directly through the media library (not inline on the node form) works without error.

Proposed resolution

Initialize $form['#prefix'] with the null coalescing operator before concatenating:

$form['#prefix'] = ($form['#prefix'] ?? '');
$form['#prefix'] .= $renderer->renderRoot($status_messages);

This is safe — when #prefix already exists it preserves the existing value, and when it doesn't it initializes to an empty string before appending status messages.

Remaining tasks

• Review and confirm the fix.
• Add a test case covering the AJAX upload callback when #prefix is not set on the resolved element.

User interface changes

None.

Introduced terminology

None.

API changes

None.

Data model changes

None.

Release notes snippet

Fixed a PHP warning ("Undefined array key #prefix") in ManagedFile::uploadAjaxCallback() that could cause intermittent AJAX file upload failures when file widgets are nested within media reference fields on entity forms.