Model code optimisation brakes import

[schel4ok]

I found that many of my models do the same things and I decided to do Models optimisation. So I moved all similar functions to trait.

CategoryTrait.php

    public function __construct()    {    $this->model = app($this->getModelClass());    }

    abstract protected function getModelClass();

    public function items() { return $this->hasMany($this->model, 'category_id'); }

NewsCategory.php

use App\Models\News as ItemModel;
use Illuminate\Database\Eloquent\Model;
use App\Models\ModelTraits\CategoryTrait;

class NewsCategory extends Model
{
    use CategoryTrait;
    protected $model;
    protected function getModelClass()    {        return ItemModel::class;    }
}

In general all is OK. Web-site works in the same way. But import from excel is not working anymore.
When I have code of the Model like above excel import gives me following error
SQLSTATE[23000]: Integrity constraint violation: 1062 Duplicate entry '' for key 'news_categories_slug_unique' (SQL: insert into news_categories () values ())

And in news_categories table I can see one row filled with NULL values

If I remove unique restriction from slug field then news_categories table will be seeded by the number of rows, which I have in excel, but instead of values from excel everywhere will be NULL

So at the moment I decided to return back to normal Model structure.
Just interesting if someone could explain to me why this happened.