Dynamic morphToMany relation through macro

I want to dynamically add a morphToMany Relation between models (e.g. User, Attachment, ...) and a File Model based in a File Package. The ServiceProvider should build the relation.

How do I do that ?

Here's the code I've tried already:

$fileables = config('topredmedia-filemanager.fileable');

foreach ((array)$fileables as $modelType) {
  if (class_exists($modelType)) {
    $model = App::makeWith($modelType);

    // Add Relation to all specified models in config
    Builder::macro('files', function () use ($model) {
      return $model->morphToMany(File::class, 'fileable', 'trm_fileables');
    });
  }
}

BTW: When I run this code but with a morphMany relation everything works fine.

When I execute $model->files()->sync([1, 2, 3]) in a controller the pivot table (file_id, fileable_id, fileable_type) should be filled with the right data.

But when I run it I get the following error: "SQLSTATE[23000]: Integrity constraint violation: 1048 Column 'fileable_id' cannot be null (SQL: insert into trm_fileables (file_id, fileable_id, fileable_type) values (3, , Path\To\Model\User))"

via Chebli Mohamed