Call to a member function connection() on null when testing

Hello everyone,

Here's my setup: Laravel v8.83 with Sail v1.15.0 and PHPUnit v9.5.21. IDE: PhpStorm where test/php configurations are set to the remote interpreter, So I'm using Sail's PHP/PHPUnit versions and etc.

I'm trying to test registration, which requires data from the DB.

Let's take an example of the Grade model I have:

<?php

namespace App\Models;

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

class Grade extends Model
{
    use HasFactory;

    protected $fillable = ['label'];

    public function users(): HasMany
    {
        return $this->hasMany(User::class);
    }
}

Related factory:

namespace Database\Factories;

use App\Models\Grade;
use Faker\Provider\Lorem;
use Illuminate\Database\Eloquent\Factories\Factory;

class GradeFactory extends Factory
{
    protected $model = Grade::class;

    public function definition(): array
    {
        $this->faker->addProvider(new Lorem($this->faker));
        return [
            'label' => $this->faker->word,
            'active' => random_int(0, 1),
            'order' => $this->faker->randomNumber(),
            'created_at' => now(),
            'updated_at' => now(),
        ];
    }
}

My phpunit.xml:

<?xml version="1.0" encoding="UTF-8"?>
<phpunit xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:noNamespaceSchemaLocation="./vendor/phpunit/phpunit/phpunit.xsd"
         bootstrap="vendor/autoload.php"
         colors="true"
>
    <testsuites>
        <testsuite name="Unit">
            <directory suffix="Test.php">./tests/Unit</directory>
        </testsuite>
        <testsuite name="Feature">
            <directory suffix="Test.php">./tests/Feature</directory>
        </testsuite>
    </testsuites>
    <coverage processUncoveredFiles="true">
        <include>
            <directory suffix=".php">./app</directory>
        </include>
    </coverage>
    <php>
        <server name="APP_ENV" value="testing"/>
        <server name="BCRYPT_ROUNDS" value="4"/>
        <server name="CACHE_DRIVER" value="array"/>
        <server name="DB_CONNECTION" value="sqlite"/>
        <server name="DB_DATABASE" value=":memory:"/>
        <server name="MAIL_MAILER" value="array"/>
        <server name="QUEUE_CONNECTION" value="sync"/>
        <server name="SESSION_DRIVER" value="array"/>
        <server name="TELESCOPE_ENABLED" value="false"/>
    </php>
</phpunit>

And RegistrationTest:

<?php

namespace Tests\Feature;

use App\Models\Grade;
use Tests\TestCase;

class RegistrationTest extends TestCase
{
    protected function setUp(): void
    {
        Grade::factory()->count(3)->make();

        parent::setUp();
    }

    /** @test */
    public function user_can_see_registration_page()
    {
        $response = $this->get('/registration');

        $response->assertStatus(200);
    }
}

I'm getting Error : Call to a member function connection() on null error:

[docker://sail-8.0/app:latest/]:php /opt/project/vendor/phpunit/phpunit/phpunit --configuration /opt/project/phpunit.xml --filter "/(Tests\Feature\RegistrationTest::user_can_see_registration_page)( .*)?$/" --test-suffix RegistrationTest.php /opt/project/tests/Feature --teamcity
Testing started at 13:45 ...
PHPUnit 9.5.21 #StandWithUkraine


Error : Call to a member function connection() on null
 /opt/project/vendor/laravel/framework/src/Illuminate/Database/Eloquent/Model.php:1653
 /opt/project/vendor/laravel/framework/src/Illuminate/Database/Eloquent/Model.php:1619
 /opt/project/vendor/laravel/framework/src/Illuminate/Database/Eloquent/Concerns/HasAttributes.php:1401
 /opt/project/vendor/laravel/framework/src/Illuminate/Database/Eloquent/Concerns/HasAttributes.php:1347
 /opt/project/vendor/laravel/framework/src/Illuminate/Database/Eloquent/Concerns/HasAttributes.php:919
 /opt/project/vendor/laravel/framework/src/Illuminate/Database/Eloquent/Model.php:431
 /opt/project/vendor/laravel/framework/src/Illuminate/Database/Eloquent/Model.php:212
 /opt/project/vendor/laravel/framework/src/Illuminate/Database/Eloquent/Factories/Factory.php:678
 /opt/project/vendor/laravel/framework/src/Illuminate/Database/Eloquent/Factories/Factory.php:387
 /opt/project/vendor/laravel/framework/src/Illuminate/Database/Eloquent/Concerns/GuardsAttributes.php:157
 /opt/project/vendor/laravel/framework/src/Illuminate/Database/Eloquent/Factories/Factory.php:392
 /opt/project/vendor/laravel/framework/src/Illuminate/Database/Eloquent/Factories/Factory.php:370
 /opt/project/vendor/laravel/framework/src/Illuminate/Database/Eloquent/Factories/Factory.php:371
 /opt/project/tests/Feature/RegistrationTest.php:18

The same is when running the sail test command:

• Tests\Feature\Auth\RegistrationTest > user can see registration page
   Error 

  Call to a member function connection() on null

  at vendor/laravel/framework/src/Illuminate/Database/Eloquent/Model.php:1653
    1649▕      * @return \Illuminate\Database\Connection
    1650▕      */
    1651▕     public static function resolveConnection($connection = null)
    1652▕     {
  ➜ 1653▕         return static::$resolver->connection($connection);
    1654▕     }
    1655▕ 
    1656▕     /**
    1657▕      * Get the connection resolver instance.

      +11 vendor frames 

What am I doing wrong here? I was searching the whole internet and everywhere I see suggestions to replace PHPUnit\Framework\TestCase with Tests\TestCase which I'm already using. I'm already researching this for two days.