Slim app got this error:

PHP Fatal error:  Uncaught TypeError: App\ContactController::__construct(): 
Argument #1 ($mailer) must be of type App\MailerService, null given, called in 
/www/slim-agency/vendor/slim/slim/Slim/CallableResolver.php on line 146 and defined in /www/slim-agency/src/ContactController.php:15

this is line 15 ContactController.php:
	public function __construct(MailerService $mailer)

// src/ContactController.php
namespace App;

use App\MailerService;

use Psr\Http\Message\ResponseInterface;
use Psr\Http\Message\ServerRequestInterface;
use HTMLPurifier;

class ContactController
{
	private MailerService $mailer;
	
	public function __construct(MailerService $mailer)
	{
		$this->mailer = $mailer;
	}
	
	public function sendEmail(ServerRequestInterface $request, ResponseInterface $response): ResponseInterface
	{
		$data = json_decode($request->getBody()->getContents(), true);
		
		// Log the contact form data for debugging (ensure sensitive data is handled appropriately)
		error_log('Contact Form Data: ' . print_r($data, true));
		
		$name = $data['name'] ?? null;
		$email = $data['email'] ?? null;
		$messageText = $data['message'] ?? null;
		if (!$name || !$email || !$messageText)
		{
			$response->getBody()->write(json_encode(['status' => 'error', 'message' => 'Invalid input']));
			return $response->withStatus(400)->withHeader('Content-Type', 'application/json');
		}
		
		// Sanitize input using HTMLPurifier and built-in PHP functions
		$purifier = new HTMLPurifier();
		$name = $purifier->purify(trim($data['name']));// Remove HTML (if any) and trim whitespace
		$email = filter_var(trim($data['email']), FILTER_SANITIZE_EMAIL);// Sanitize email
		$message = $purifier->purify(trim($data['message']));
		
		// Validate the sanitized email
		if (!filter_var($email, FILTER_VALIDATE_EMAIL))
		{
			$response->getBody()->write(json_encode(['status' => 'error', 'message' => 'Invalid email address']));
			return $response->withStatus(400)->withHeader('Content-Type', 'application/json');
		}
		
		try
		{
			// Attempt to send the email
			$this->mailer->send($email, 'New Contact Form Submission', "Name: $name\nMessage: $messageText");
			
			$response->getBody()->write(json_encode(['status' => 'success', 'message' => 'Email sent successfully!']));
			return $response->withStatus(200)->withHeader('Content-Type', 'application/json');
		}
		catch (\Exception $e)
		{
			// Log the error for debugging
			error_log('Email sending failed: ' . $e->getMessage());
			
			$response->getBody()->write(json_encode(['status' => 'error', 'message' => 'Error sending email: ' . $e->getMessage()]));
			return $response->withStatus(500)->withHeader('Content-Type', 'application/json');
		}
	}
}

this is the rest of the app:

// src/Dependencies.php
namespace App;

use App\MailerService;
use App\ContactController;

use DI\Container;

return function (Container $container) {

	// Register the Mailer service
	$container->set(MailerService::class, \DI\autowire(MailerService::class));

	// Register the ContactController
	$container->set(ContactController::class, \DI\autowire(ContactController::class));
};

// src/Mailer.php
namespace App;

use Symfony\Component\Mailer\Mailer;
use Symfony\Component\Mailer\Transport;
use Symfony\Component\Mime\Email;
use Symfony\Component\Mailer\Transport\AbstractTransport;
use Symfony\Component\Mailer\SentMessage;

class MailerService
{
	private Mailer $mailer;
	
	public function __construct() {
		
		// Load the environment variables
		$env = $_ENV['APP_ENV'] ?: 'production';
		$baseDir = $_ENV['BASE_DIR'] ?? '';
		$tmpDir = $_ENV['TMP_DIR'] ?? $baseDir . 'tmp/';
		$tmpDir = rtrim($tmpDir, '/') . '/';// Ensure TMP_DIR ends with a trailing slash.
		if (!is_dir($tmpDir))
		{
			mkdir($tmpDir, 0755, true);// Create the directory with 0755 permissions
		}
		if ($env === 'development')
		{
			$transport = new class($tmpDir) extends AbstractTransport
			{
				private string $logDirectory;
				
				public function __construct(string $logDirectory)
				{
					$this->logDirectory = rtrim($logDirectory, '/') . '/';
				}
				
				protected function doSend(SentMessage $message): void
				{
					// Create a filename based on the current timestamp
					$filename = $this->logDirectory . 'email_' . date('Ymd_His') . '.eml';
					file_put_contents($filename, (string)$message);
				}
				
				public function __toString(): string
				{
					return 'file://' . $this->logDirectory;
				}
			};
		}
		else {
			// Use SMTP for production
			$dsn = $_ENV['MAILER_DSN'];// e.g., smtp://username:<PRESIDIO_ANONYMIZED_EMAIL_ADDRESS>:port
			$transport = Transport::fromDsn($dsn);
		}
		
		$this->mailer = new Mailer($transport);
	}
	
	public function send(string $to, string $subject, string $body): void
	{
		$message = (new Email())
		->from('your_email@example.com')// Change to a valid sender email
		->to($to)
		->subject($subject)
		->text($body);
		
		$this->mailer->send($message);
	}
}

// src/routes.php
namespace App;

use App\ContactController;

use Slim\App;
use Psr\Http\Message\ResponseInterface;
use Psr\Http\Message\ServerRequestInterface;

return function (App $app)
{
	
	// Send Email Api
	$app->post('/send-email', [ContactController::class, 'sendEmail']);
};

index.php

// public/index.php
ini_set('log_errors', 'On');
ini_set('error_log', __DIR__ . '/../app_errors.log');// Log file at the root

require __DIR__ . '/../vendor/autoload.php';

use Dotenv\Dotenv;
use DI\Container;
use DI\Bridge\Slim\Bridge;
use Slim\Factory\AppFactory;
use Slim\Psr7\Response;// Necessary if you are directly creating instances of Slim's Response class.

$dotenv = Dotenv::createImmutable(__DIR__ . '/../', '.env');
$dotenv->load();

$container = new Container();
(require __DIR__ . '/../src/Dependencies.php')($container);

$app = Bridge::create($container);
$app = AppFactory::create();
(require __DIR__ . '/../src/Routes.php')($app);

$app->add(function ($request, $handler) {
	try
	{
		return $handler->handle($request);
	}
	catch (\Exception $e)
	{
		error_log($e->getMessage());
		
		$response = new Response();
		$response->getBody()->write(json_encode(['status' => 'error', 'message' => 'An internal server error occurred.']));
		return $response->withStatus(500)->withHeader('Content-Type', 'application/json');
	}
});

$app->run();