SimpleJSON returns null - simplejson

Question

i have an error in c# of NullReferenceException: Object reference not set to an instance of an object
SimpleJSON.JSONNode.Parse (System.String aJSON) (at Assets/Scenes/scripts/SimpleJSON.cs:553)
I found this post in the link below but i didn't understand the solution
SimpleJSON returns null when handling a call from Postmen
Can any one explain it for me, please?
the error is :
NullReferenceException: Object reference not set to an instance of an object
SimpleJSON.JSONNode.Parse (System.String aJSON) (at Assets/Scenes/scripts/SimpleJSON.cs:553)
SimpleJSON.JSON.Parse (System.String aJSON) (at Assets/Scenes/scripts/SimpleJSON.cs:1363)
Control.Update () (at Assets/Control.cs:123)
knowing that i'm sending json data using websocket and i'm receiving it in the console unity. i use simpleJSON script to deserialize the data.
here where the error is provided in line JSONNode root = JSON.Parse(last_message); :
private void Update()
{
if (ws == null)
{
return;
}
List<GameObject> object_to_keep = new List<GameObject>();
JSONNode root = JSON.Parse(last_message);
JSONNode Face = root["streams"][0]["objects"][0];
{
int id = Face["mono"]["id"].AsInt;
JSONNode position = Face["mono"]["position"];
Vector3 pos = new Vector3(position["x"].AsFloat, position["y"].AsFloat, position["z"].AsFloat);
GameObject mono = GetMonoObject();
WintualFaceController win_controller = mono.GetComponent<WintualFaceController>();
win_controller.face_id = id;
if(mono)
{
mono.transform.localPosition = pos;
object_to_keep.Add(mono);
}
else
print("Mono not found");
}
}
The error in SImpleJSON script wan in line: while (i < aJSON.Length)
public static JSONNode Parse(string aJSON)
{
Stack<JSONNode> stack = new Stack<JSONNode>();
JSONNode ctx = null;
int i = 0;
StringBuilder Token = new StringBuilder();
string TokenName = "";
bool QuoteMode = false;
bool TokenIsQuoted = false;
while (i < aJSON.Length)
{
switch (aJSON[i])
{
case '{':
if (QuoteMode)
{
Token.Append(aJSON[i]);
break;
}
stack.Push(new JSONObject());
if (ctx != null)
{
ctx.Add(TokenName, stack.Peek());
}
TokenName = "";
Token.Length = 0;
ctx = stack.Peek();
break;
case '[':
if (QuoteMode)
{
Token.Append(aJSON[i]);
break;
}
stack.Push(new JSONArray());
if (ctx != null)
{
ctx.Add(TokenName, stack.Peek());
}
TokenName = "";
Token.Length = 0;
ctx = stack.Peek();
break;
case '}':
case ']':
if (QuoteMode)
{
Token.Append(aJSON[i]);
break;
}
if (stack.Count == 0)
throw new Exception("JSON Parse: Too many closing brackets");
stack.Pop();
if (Token.Length > 0 || TokenIsQuoted)
{
ParseElement(ctx, Token.ToString(), TokenName, TokenIsQuoted);
TokenIsQuoted = false;
}
TokenName = "";
Token.Length = 0;
if (stack.Count > 0)
ctx = stack.Peek();
break;
case ':':
if (QuoteMode)
{
Token.Append(aJSON[i]);
break;
}
TokenName = Token.ToString();
Token.Length = 0;
TokenIsQuoted = false;
break;
case '"':
QuoteMode ^= true;
TokenIsQuoted |= QuoteMode;
break;
case ',':
if (QuoteMode)
{
Token.Append(aJSON[i]);
break;
}
if (Token.Length > 0 || TokenIsQuoted)
{
ParseElement(ctx, Token.ToString(), TokenName, TokenIsQuoted);
TokenIsQuoted = false;
}
TokenName = "";
Token.Length = 0;
TokenIsQuoted = false;
break;
case '\r':
case '\n':
break;
case ' ':
case '\t':
if (QuoteMode)
Token.Append(aJSON[i]);
break;
case '\\':
++i;
if (QuoteMode)
{
char C = aJSON[i];
switch (C)
{
case 't':
Token.Append('\t');
break;
case 'r':
Token.Append('\r');
break;
case 'n':
Token.Append('\n');
break;
case 'b':
Token.Append('\b');
break;
case 'f':
Token.Append('\f');
break;
case 'u':
{
string s = aJSON.Substring(i + 1, 4);
Token.Append((char)int.Parse(
s,
System.Globalization.NumberStyles.AllowHexSpecifier));
i += 4;
break;
}
default:
Token.Append(C);
break;
}
}
break;
default:
Token.Append(aJSON[i]);
break;
}
++i;
}
this is my json:
{
"cmd": "data",
"frame_time": "2021-06-30T09:04:16.464836+00:00",
"start_time": "2021-06-30T07:51:33.452079+00:00",
"streams": [
{
"name": "usb-0001:012.0-3",
"objects": [
{
"name": "face",
"mono": {
"id": "190",
"position": {
"x": "-0.012259",
"y": "0.059731",
"z": "0.707695"
}
},
"multi": [
{
"id": "190",
"position": {
"x": "-0.012263",
"y": "0.059753",
"z": "0.707151"
}
}
]
}
]
}
]
}