Visual Studio Debug problem: Using LitJson in Unity, If I watch a json_array by json_array[], it won't show and raise an error

Question

I'm using LitJson in Unity, debug with Visual Studio.
As shown in the picture.
I can get an item from json_array by JsonData item0 = json_array[0];
But if I watch json_array[0], it raise an error.

This is the second time I come up with a problem where code runs right but debug runs wrong.
I don't know anything about how debug works.
Is there anything I can do or anthing I can check if "code runs right but debug runs wrong" happens again?
(The first time is Does C# Nullable have bug in Unity?)