MAMP server is not responding to api post request correctly: Internal Server Error 500: „Call to a member function search() on null“

I am following a course about shopware 6 on how to display data throughout the storefront, which would be provided with an API controller. I already checked the code multiple times with the tutorial code and I can not find any mistakes. The API client Insomnia is able to recognize the API route, nevertheless, the server responds with an internal server error.

"detail": "Call to a member function search() on null",
  "meta": {
    "trace": [
        "file": "\/Applications\/MAMP\/htdocs\/shopware6_testing\/custom\/plugins\/SwagShopFinder\/src\/Core\/Api\/DemoDataController.php",
        "line": 63,
        "function": "getActiveCountry",

In fact, I am quite new to Symfony and I am not able to read and understand the code fluently. I assume there is something wrong with the method getActiveCountry, but I am not sure how to engage this.

The response that I should be getting is 204 HTTP_NO_CONTENT, throughout the function generate().

 public function generate(Context $context): Response
    $faker = Factory::create();
    $country = $this->getActiveCountry($context);
    $data = [];
    for ($i = 0; $i < 50; $i++) {
        $data[] = [
            'id' => Uuid::randomHex(),
            'active' => true,
            'name' => $faker->name,
            'street' => $faker->streetAddress,
            'postCode' => $faker->postcode,
            'city' => $faker->city,
            'countryId' => $country->getId(),
    $this->shopFinderRepository->create($data, $context);

    return new Response('', Response::HTTP_NO_CONTENT);

Domain Route (proposed by the shopware 6 Docs.):

/** * @RouteScope(scopes={"api"}) */

API Route:
* @Route("/api/v{version}/_action/swag-shop-finder/generate", name="api.custom.swag_shop_finder.generate", methods={"POST"})

The endpoint /generate should trigger the function generate(), which continually generates 50 shops stored in an array and give the server Respond 204 HTTP_NO_CONTENT.

The API controller is registered in a services.xml and a separate routes.xml file:

<service id="SwagShopFinder\Core\Api\DemoDataController" public="true">
    <argument id="country.repository" type="service" />
    <argument id="swag_shop_finder.repository" type="service" />
    <call method="setContainer">
        <argument type="service" id="service_container" />

routes.xml file:

 <import resource="../../**/Api/*Controller.php" type="annotation" />

If more information is needed I am happy to share it!