See mridula's answer for why you are getting null. But if you want to directly convert the json string to dictionary you can try following code snippet.
Dictionary<string, object> values =
JsonConvert.DeserializeObject<Dictionary<string, object>>(json);
与恶龙缠斗过久,自身亦成为恶龙;凝视深渊过久,深渊将回以凝视…