how to pass null value in the json while constructing it

I have the below code where I am assigning values coming from a string to a key value in here, for ex below I am assigning obj[_id] to id.But there is possibility that my jstr do not contain _id key and in that case I would like to assign value "null" to my id. How can I do that in below code?

    foreach (JObject obj in JArray.Parse(jStr))
                    {
                        var options = new JsonTranslatorOptions(
                        kind: "ENTITY",
                        id: obj["_id"],
                        type: "EQUIPMENT",
                        version: "data-4-build-606");
                        var translator = new JsonTranslator(options);
                        output = translator.Translate(obj);
                    }
internal class JsonTranslatorOptions
    {
        public JsonTranslatorOptions(
            string kind,
            JToken id,
            string type,
            string version)
        {
            Kind = kind;
            Id = id.ToString();
            Type = type;
            Version = version;
        }

        public string Kind { get; }
        public string Id { get; }
        public string Type { get; }
        public string Version { get; }
    }

Are you accessing id.ToString() when id is null ?

try this:

    public JsonTranslatorOptions(
        string kind,
        JToken id,
        string type,
        string version)
    {
        Kind = kind;
        Id = id?.ToString(); // null check
        Type = type;
        Version = version;
    }