Why am I getting InaccessibleObjectException when writing to JSON file - exception

Question

I'm using Gson to manage my Json file. I have a User class which looks like this:
public class User {
private StringProperty username;
private StringProperty password;
...
public User() {}
public User(String user, String pass, ...) {
this.username = new SimpleStringProperty(user);
this.password = new SimpleStringProperty(pass);
...
}
public String getUsername() {
return username.get();
}
public void setUsername(String username) {
this.username.set(username);;
}
...
}
And this is how I add a User to the Json file
public static boolean addUser(User user) throws IOException{
String users = new String(Files.readAllBytes(Paths.get("users.json")));
List<User> userList = getUsers(users);
if (userList.contains(user)) return false;
userList.add(user);
Writer writer = new FileWriter("users.json");
Gson gson = new GsonBuilder().create();
gson.toJson(userList, writer);
writer.close();
return true;
}
gson.toJson(userList, writer) is throwing this error:
Exception in thread "JavaFX Application Thread" java.lang.reflect.InaccessibleObjectException: Unable to make field private final java.lang.Object javafx.beans.property.SimpleStringProperty.bean accessible: module javafx.base does not "opens javafx.beans.property" to unnamed module #4bf59938
I know it has something to do with the StringProperty attributes, but I don't know what's wrong.