fun sendMessageToClient(client: Client?, message: String?, mailer: Mailer) {
    if (client == null || message == null) return
    var personalInfo = client.personalInfo;
    if (personalInfo == null) return;
    var email = personalInfo.email;
    if (email == null) return;
    mailer.sendMessage(email, message);
}

class Client (val personalInfo: PersonalInfo?)
class PersonalInfo (val email: String?)
interface Mailer {
    fun sendMessage(email: String, message: String)
}