void setServer(Server server) {
        if (this.server != null) {
            throw new IllegalStateException("ServerConfig cannot be used for more than one Server.");
        }
        this.server = requireNonNull(server, "server");
    }
Initialized later by {@link Server} via {@link #setServer(Server)}.
core/src/main/java/com/linecorp/armeria/server/DefaultServerConfig.java