Trying to open Images with jSon Object shows unreliable behavior

Processing
13

I still don’t know why the above code behave the way it does. But in the meantime I experimented with multi threading (never worked with it until now, but heard it somewhere) and it seems to work:

import processing.serial.*;

String myString = null;
Serial myPort;  // The serial port
PImage imgGreen;
PImage imgRed;
PImage wallpaper;
boolean XFflag = false; 
boolean jsonFlag = false;

void setup() {
  fullScreen();
  myPort = new Serial(this, Serial.list()[0], 9600);
  myPort.clear();
  imgGreen = loadImage("greenEggSmall.png");
  imgRed = loadImage("redEggSmall.png");
  wallpaper = loadImage("Wallpaper4.jpg");
}

void draw() {
  background(wallpaper);
  thread("jSonProcessing");
  image(XFflag ? imgGreen : imgRed, 1030, 665);
}

void jSonProcessing() {
  while (myPort.available() > 0) {
    String myString = myPort.readStringUntil('}');
    if (myString != null) {
      if (myString.startsWith("{")) {
        jsonFlag = true;
      } else {
        jsonFlag = false;
      }
      if (jsonFlag == true) {
        try {
          JSONObject json = JSONObject.parse(myString);
          String btnXForward = json.getString("XF");
          XFflag =  btnXForward.equals("1");  //Updates XF field
        } 
        catch (Exception e) {
          println("An error occured, while trying to parse the jSon object.");
        }
      }
    }
  }
}