Error: Runtime exited without providing a reason Runtime.ExitError

I was trying to get this working using Kotlin but I keep getting this: Error: Runtime exited without providing a reason Runtime.ExitError.

My handler is setup like this

class SimpleHandler: RequestHandler {
val logger = LoggerFactory.getLogger(SimpleHandler::class.java)
val jsonMapper = jacksonObjectMapper()
var responseFromClient: String = ""

override fun handleRequest(input: APIGatewayProxyRequestEvent?, context: Context?): APIGatewayProxyResponseEvent {
logger.info("APIGatewayProxyRequestEvent received: \n ${jsonMapper.writeValueAsString(input)}")

}

}

I don't have a main class as the java example. I noticed that when I added a main class I get a different error: Request Timeout but maybe is because is trying to wait for and answer from main execution. Please advice.