Hi, i'm working on a custom notification blueprint for Flarum, and I've run into an issue with the getSubjectModel method.

In my use case, the subjectId is a simple integer representing a category or type, not an actual model (like User or Post). However, getSubjectModel seems to require a valid class name, and leaving it as an empty string ('') or using a placeholder value (like 'custom_subject') results in errors:

If I leave getSubjectModel as an empty string:

   public static function getSubjectModel(): string
   {
       return '';
   }

This causes the following error in the browser:

   Uncaught TypeError: Cannot read properties of null (reading 'type')

If I try to return a placeholder string (e.g., 'custom_subject'):

   public static function getSubjectModel(): string
   {
       return 'custom_subject';
   }

It results in:

   Error: Class "custom_subject" not found

What I Want to Achieve:

subjectId is just an integer and doesn't correspond to any database model.
I need to implement getSubjectModel in a way that works with Flarum's notification system without linking it to an actual model.

My Current Setup:

Controller: Passes subjectId (integer) to the CustomNotificationBlueprint.

Blueprint: Includes subjectId in the getData method:

   public function getData(): array
   {
       return [
            'type' => $this->notificationType,
            'message' => $this->message,
            'fromUserId' => $this->fromUser?->id,
            'subjectId' => $this->subjectId,
            'unique' => (string) Str::orderedUuid(),
       ];
   }

Question:
How can I implement getSubjectModel properly when my subjectId is not linked to any model? Is there a way to bypass the need for a model entirely in this context, or should I create a "dummy" model class just to satisfy this requirement?

Any advice or guidance would be greatly appreciated!