NullReference Exception while serializing JSON string to Object in Windows Application

I am having an error at the line "obj.flag" saying my object is null, also I don't try to read an empty file I handled it with if-else. I see no problem with my  (de)serialization neither. I tried ignoring null's like;

MissingMemberHandling = MissingMemberHandling.Ignore, NullValueHandling = NullValueHandling.Ignore

Those kind of codes didn't work too. I am stuck, can someone please help me

        bool portStateIsWritten;
        //bool var = Class1.flag;
        //FILENAME
        string input = Interaction.InputBox("Enter a serial number", "TEST", "Default", -1, -1);
        //DEFAULT PATH ROOT
        String root = @".\\";
        string path_combined;
        FlagClass obj = new FlagClass();

        public UI()
        {
            InitializeComponent();
            foreach (string s in SerialPort.GetPortNames())
            {
                cbxPort.Items.Add(s);
                cbxPort.SelectedIndex = 0;
            }
            input += ".txt";
            //default path + new filename
            path_combined = Path.Combine(root, input);

            var settings = new JsonSerializerSettings
            {
                MissingMemberHandling = MissingMemberHandling.Ignore,
                NullValueHandling = NullValueHandling.Ignore
            };
            if (!File.Exists(path_combined))
            {

                using (var stream = File.Create(path_combined))
                {
                    if (new FileInfo(path_combined).Length > 0)
                    {
                        //flag situation
                        string json2 = File.ReadAllText(path_combined);
                        FormatJson(json2);
                        obj = JsonConvert.DeserializeObject<FlagClass>(json2, settings);

                    }
                }

            }
            else
            {
                //flag situation
                string json2 = File.ReadAllText(path_combined);
                FormatJson(json2);
                obj = JsonConvert.DeserializeObject<FlagClass>(json2, settings);


            }
        }

        private static string FormatJson(string json)
        {
            dynamic parsedJson = JsonConvert.DeserializeObject(json);
            return JsonConvert.SerializeObject(parsedJson, Formatting.Indented);
        }
        void Serialize()
        {
            string json = JsonConvert.SerializeObject(obj, Formatting.Indented, new JsonSerializerSettings { NullValueHandling = NullValueHandling.Ignore });
            File.WriteAllText(path_combined, json);
        }        
        
        private  void btn_test1_Click(object sender, EventArgs e)
        {  obj.flag = true;
            //serialize
            Serialize();
        }