Serialize and deserialize with JSON writing to “.txt” problem

EDIT:
I fix the previous errors,
I understand my problem with the code is related to writing a blank file; when I click my button to set values to write a file;
It says my "c1" object is empty.

System.NullReferenceException: 'Object reference not set to an instance of an object.'

at this line since I click this button:

c1.flag = true

My file is blank I know, the buttons are made to fill my text when I press them. What does it have to do with c1, it is just an object to provide communication with my other class.

-I create a new file with given path, then write the json object to a blank txt.

What I have tried:

[Serializable]

    public partial class Form1 : Form
    {
        
        //FILENAME
        string input = Interaction.InputBox("Enter a serial number", "TEST", "Default", -1, -1);
        //DEFAULT PATH ROOT
        String root = @"C:\Users\Dell\source\repos\SaveReloadDeneme\SaveReloadDeneme\bin\Debug";

        Class1 c1 = new Class1();//holds method's attribute flags
        Class1 c2 = new Class1(); //holds method flags whether the used or not

        string path_combined;
        public Form1()
        {
            InitializeComponent();
            input += ".txt";
            //default path + new filename
            path_combined = Path.Combine(root, input);


            if (!File.Exists(path_combined))
            {
                File.Create(path_combined);
                
            }


            //flag situation
            string json2 = File.ReadAllText(path_combined);
            FormatJson(json2);
            c1 = JsonConvert.DeserializeObject<Class1>(json2);
            

            //Method used or not
            string json = File.ReadAllText(path_combined);
            FormatJson(json);
            c2 = JsonConvert.DeserializeObject<Class1>(json);

        }

        private static string FormatJson(string json)
        {
            dynamic parsedJson = JsonConvert.DeserializeObject(json);
            return JsonConvert.SerializeObject(parsedJson, Formatting.Indented);
        }

        // SET TRUE
        private void Button1_Click(object sender, EventArgs e)
        {

            c1.flag = true;
            c1.flag2 = true;
            c2.M1 = true;
            string json = JsonConvert.SerializeObject(c1, Formatting.Indented);
            File.WriteAllText(path_combined, json);

            string json2 = JsonConvert.SerializeObject(c2, Formatting.Indented);
            File.WriteAllText(path_combined, json2);

        }

[Serializable]
    
    class Class1
    {
        [JsonProperty(PropertyName = "flag")]
        public bool flag { get; set; }


        [JsonProperty(PropertyName = "flag2")]
        public bool flag2 { get; set; }

        //indication whether the method is used or not
        [JsonProperty(PropertyName = "SET TRUE USED")]
        public bool M1 { get; set; }

        [JsonProperty(PropertyName = "SET FALSE USED")]
        public bool M2 { get; set; }
        public Class1()
        {
           
        }
    }

Tags: C#, JSON, Serialization