Implicitly marking parameter $method as nullable is deprecated

1

I downloaded and ran slim-framework, and got an error message:
{
“statusCode”: 500,
“error”: {
“type”: “SERVER_ERROR”,
“description”: “ERROR: DI\Definition\Resolver\ParameterResolver::resolveParameters(): Implicitly marking parameter $method as nullable is deprecated, the explicit nullable type must be used instead on line 40 in file /home/mirjr/Code/php/lessons/slim-framework/vendor/php-di/php-di/src/Definition/Resolver/ParameterResolver.php.”
}
}