Serialize and Deserialize text file Error: Object reference not set to an instance of an object

What the code does is:

I create new files in each run manually by giving names of the files to save Ex: filename= "XYZ", file is saved as "XYZ.txt". When I close and open program data is fetched from txt to show the program's previous state. It continues where it left off. 

I am having this error when I press a button;

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

for my c1 object.

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" object, it is just an object to provide communication with my other class.

    [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);
        }