test fails on member function null

Good morning friends

this time I have a easy one for you but I am not experienced enough to track the bug ...

I get the following error:

Error : Call to a member function connection() on null

<?php

namespace Tests\Unit;

use App\Models\Offer;
use App\Models\User;
use Illuminate\Foundation\Testing\RefreshDatabase;
use PHPUnit\Framework\TestCase;

class OfferTest extends TestCase
{
    use RefreshDatabase;

//   protected $offer;

//    public function setUp(): void
//    {
//        parent::setUp(); // TODO: Change the autogenerated stub
//        $this->offer = Offer::factory()->create();
//    }

    /** @test */
    function an_offer_has_a_creator()
    {
        $offer = Offer::factory()->create();
        $offer->assertInstanceOf(User::class, $this->offer->user);
    }

}

<?php

use Illuminate\Database\Migrations\Migration;
use Illuminate\Database\Schema\Blueprint;
use Illuminate\Support\Facades\Schema;

class CreateOffersTable extends Migration
{
    /**
     * Run the migrations.
     *
     * @return void
     */
    public function up()
    {
        Schema::create('offers', function (Blueprint $table) {
            $table->id();

            $table->unsignedBigInteger('user_id');
            $table->foreign('user_id')->references('id')->on('users');
            $table->string('Name');

            $table->decimal('latitude', 8, 6);
            $table->decimal('longitude', 9, 6);
            $table->timestamps();
        });
    }

    /**
     * Reverse the migrations.
     *
     * @return void
     */
    public function down()
    {
        Schema::dropIfExists('offers');
    }
}

<?php

namespace Database\Factories;

use App\Models\Offer;
use Illuminate\Database\Eloquent\Factories\Factory;
use Illuminate\Support\Facades\Hash;
use Illuminate\Support\Str;

class OfferFactory extends Factory
{
    /**
     * The name of the factory's corresponding model.
     *
     * @var string
     */
    protected $model = Offer::class;

    public function definition ()
    {
        return [
            'Name' => 'Breath Work',

            'latitude' => 51.567952 ,
            'longitude' => 8.080428 ,
        ];
    }
}

<?php

namespace App\Models;

use Illuminate\Database\Eloquent\Factories\HasFactory;
use Illuminate\Database\Eloquent\Model;

class Offer extends Model
{
    use HasFactory;

public function user()
    {
        return $this->belongsTo(User::class);
    }
}

also a side question ... ist the without exception handling not done anymore?