Error - com.google.firebase.database.DatabaseException: Can't convert object of type java.lang.String

I am getting error

"com.google.firebase.database.DatabaseException: Can't convert object of type java.lang.String to type krunoslavkovac.ferit.test.Add"

And can't figure it out why is it happening.

I've searched many solutions here but still didn't figure it out.

MAIN ACTIVITY :

public class MainActivity extends AppCompatActivity {
    EditText et_task ;
    Button btn_add;
    RecyclerView recycler_view;

    //Firebase
    FirebaseDatabase database;
    DatabaseReference dbRef ;


    FirebaseRecyclerOptions<Add> options;
    FirebaseRecyclerAdapter<Add, MyRecyclerViewHolder> adapter;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        et_task = (EditText) findViewById(R.id.et_task);
        btn_add = (Button) findViewById(R.id.btn_add);
        recycler_view = (RecyclerView) findViewById(R.id.recycler_view);
        recycler_view.setLayoutManager(new LinearLayoutManager(this));

        database = FirebaseDatabase.getInstance();
        dbRef = database.getReference("Test");

        btn_add.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                addTask();
            }
        });
        displayTask();
    }

    private void addTask(){
        String task = et_task.getText().toString();
        Add add = new Add(task);

        dbRef.push().setValue(task);
        adapter.notifyDataSetChanged();
    }

    private void displayTask() {
        options =
                new FirebaseRecyclerOptions.Builder<Add>()
                        .setQuery(dbRef,Add.class)
                        .build();

        adapter =
                new FirebaseRecyclerAdapter<Add, MyRecyclerViewHolder>(options) {
                    @Override
                    protected void onBindViewHolder(@NonNull MyRecyclerViewHolder holder, int position, @NonNull Add model) {
                        holder.tv_task.setText(model.getTask());
                    }

                    @NonNull
                    @Override
                    public MyRecyclerViewHolder onCreateViewHolder(@NonNull ViewGroup viewGroup, int i) {
                        View itemView = LayoutInflater.from(getBaseContext()).inflate(R.layout.add_item,viewGroup, false);
                        return new MyRecyclerViewHolder(itemView);
                    }
                };
        adapter.startListening();
        recycler_view.setAdapter(adapter);
    }
}

Add class

public class Add {
    private String task;

    public Add(){

    }
    public Add(String task) {
        this.task = task;
    }


    public String getTask() {
        return task;
    }

    public void setTask(String task) {
        this.task = task;
    }
}

RecyclervViewHolder

public class MyRecyclerViewHolder extends RecyclerView.ViewHolder {

TextView tv_task;

public MyRecyclerViewHolder(@NonNull View itemView) {
    super(itemView);
    tv_task = (TextView) itemView.findViewById(R.id.tv_task);
   }
}

Database:

I want to display data from firebase.