LitJson not working

Hi I watched https://www.youtube.com/watch?v=z9cYAQhMCgE&list=WL∈dex=33 and did exactly the same but when i tried to run the game error occured. "NullReferenceException: Object reference not set to an instance of an object". These two lines from code are causing it:

             dialogue = JsonMapper.ToObject(jsonTextFile.text);
             textDisplay.text = dialogue[index][0].ToString();

What should I do?