An application that registers a core/Loader extension and returns a flat list of URLs will take down every front-end page on the site, not just its own.

The symptom

Every front page returns a 500. The log says:

TypeError: array_merge(): Argument #2 must be of type array, string given
#0 /system/Dispatcher/Front.php(188): array_merge(Array, 'https://...')

Note where the error comes from: the dispatcher, not your application. Nothing in the trace names the app responsible.

The cause

The front dispatcher merges each entry into its file list:

foreach ( $loader->js() as $js )
{
    $jsFiles = array_merge( $jsFiles, $js );
}

Each element is expected to be an array. Returning array( $url ) passes a string into array_merge() and throws before any page renders.

The fix

public function js(): array
{
    return array( array( $url ) );   // an array OF arrays
}

Why this is easy to miss

Calling js() yourself and seeing a non-empty array back is not a test. The failure only happens inside the dispatcher merge loop. Test by reproducing that loop across every installed Loader extension, or by actually requesting a page.

Recovering

If a site is already down, disabling the application restores it immediately: set app_enabled = 0 for that application and clear the datastore. No files need to be touched.

Related: Widget conventions: two language keys, ipsWidget markup, and cached blanks, The core/LiveSearch extension in Invision Community 5, The core/OutputPlugins extension: how a {tag} in a template is resolved