Sometimes when I call this code:

Widget displayImage(BuildContext context, MyBook book) {
  if (book.coverUrl != null) {
    try {
      return Image.network(book.coverUrl!,
          errorBuilder: (context, error, stackTrace) {
        return Text('No nimage');
      });
    } on Exception {
      return Text('No nImage');
    }
  } else {
    return Text('No nImage');
  }
}

I receive the message

════════ Exception caught by image resource service ══════════════════

Invalid argument(s): No host specified in
URI file:///home/alan/FlutterProjects/books/null
═══════════════════════════════════════════════════════

(file:///home/alan/FlutterProjects/books/ is my app’s path. book.coverUrl is a String.)
The online image displays properly, despite the Exception message.

What causes this? Do I need to fix it?