Exception occurd while writing and reading from Gson

i have a class name Rule and i am trying to write and read from GSON so i add Gson library (Gson-2.6.2) to my project. here is my code. but when i try to use them it returns an error( or exception). here is my code :

 public void writeOnJSON(Rule rule) {
        Gson gson = new Gson();


        String json = gson.toJson(rule);

        try {
            FileWriter writer = new FileWriter(JSON_FILE);
            writer.write(json);
            writer.close();

        } catch (IOException e) {
            e.printStackTrace();
        }

    }



public void readFromJSON() {
        Gson gson = new Gson();

        try {
        RulesList.getRuleArrayList().clear();
            System.out.println("Reading JSON from a file");
            BufferedReader br = new BufferedReader(
                    new FileReader(JSON_FILE));

            //convert the json string back to object
            Rule rule = gson.fromJson(br, Rule.class);
            RulesList.getRuleArrayList().add(rule);


        } catch (IOException e) {
            e.printStackTrace();
        }
    }

and here is the error enter image description here

Read more here: https://stackoverflow.com/questions/66273312/exception-occurd-while-writing-and-reading-from-gson