Receiving error : E/RecyclerView: No adapter attached; skipping layout (Java)

I am receiving error

> E/RecyclerView: No adapter attached; skipping layout.

I have been trying to solve this problem by referring other programmer, but did not find any solution. I am new to all this

Also, The Recycler view is not showing up.

This is my Main Activity

import androidx.appcompat.app.AppCompatActivity;
import androidx.appcompat.widget.SearchView;
import androidx.recyclerview.widget.LinearLayoutManager;
import androidx.recyclerview.widget.RecyclerView;

import android.app.SearchManager;
import android.content.Context;
import android.os.Bundle;
import android.view.Menu;
import android.view.MenuInflater;
import android.view.View;
import android.widget.ProgressBar;
import android.widget.Toast;

import java.util.List;

import retrofit2.Call;
import retrofit2.Callback;
import retrofit2.Response;

public class MainActivity extends AppCompatActivity {

    private RecyclerView recyclerView;
    private RecyclerView.LayoutManager layoutManager;
    private List<hsn> Hsn;
    private Adapter adapter;
    private ApiInterface apiInterface;
    ProgressBar progressBar;


    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);


        progressBar = findViewById(R.id.progress);
        recyclerView = findViewById(R.id.recycler);
        layoutManager = new LinearLayoutManager(this);
        recyclerView.setLayoutManager(layoutManager);
        recyclerView.setHasFixedSize(true);





        fetchhsn(""); //without keyword

    }

    public void fetchhsn (String key){
        apiInterface = ApiClient.getApiClient().create(ApiInterface.class);
        Call<List<hsn>> call = apiInterface.gethsn(key);

//        adapter = new Adapter(Hsn, MainActivity.this);


        call.enqueue(new Callback<List<hsn>>() {
            @Override
            public void onResponse(Call<List<hsn>> call, Response<List<hsn>> response) {
                adapter = new Adapter(Hsn,MainActivity.this);
                progressBar.setVisibility(View.GONE);
                recyclerView.setAdapter(adapter);
                adapter.notifyDataSetChanged();
                Hsn = response.body();

            }

            @Override
            public void onFailure(Call<List<hsn>> call, Throwable t) {
                progressBar.setVisibility(View.GONE);
                Toast.makeText(MainActivity.this,"ERROR ON :" + t.toString(), Toast.LENGTH_LONG).show();

            }
        });
    }

    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        MenuInflater inflater = getMenuInflater();
        inflater.inflate(R.menu.menu, menu);

        SearchManager searchManager = (SearchManager) getSystemService(Context.SEARCH_SERVICE);
        SearchView searchView = (SearchView) menu.findItem(R.id.search).getActionView();

        searchView.setSearchableInfo(
                searchManager.getSearchableInfo(getComponentName())
        );

        searchView.setIconifiedByDefault(false);
        searchView.setOnQueryTextListener(new SearchView.OnQueryTextListener() {
            @Override
            public boolean onQueryTextSubmit(String query) {
                fetchhsn(query);
                return false;
            }

            @Override
            public boolean onQueryTextChange(String newText) {
                fetchhsn(newText);
                return false;
            }
        });

        return true;
    }
}

This is my Adapter

import android.content.Context;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.TextView;

import androidx.annotation.NonNull;
import androidx.recyclerview.widget.RecyclerView;

import java.util.List;

public class Adapter extends RecyclerView.Adapter<Adapter.MyViewHolder> {

    private List<hsn> Hsn;
    private Context context;

    Adapter(List<hsn> HSN, Context context) {
        this.Hsn = HSN;
        this.context = context;
    }

    @NonNull
    @Override
     public MyViewHolder onCreateViewHolder( ViewGroup parent, int viewType) {
     View view = LayoutInflater.from(parent.getContext()).inflate(R.layout.activity_item, parent, false);
     return new MyViewHolder(view);
 }

        //return rootView; enter in the and in the Tab Fragment:  by biju

    @Override
    public void onBindViewHolder( MyViewHolder holder, int position) {
        holder.GSTCODE.setText(Hsn.get(position).getGSTCODE());
        holder.CODE.setText(Hsn.get(position).getCODE());
    //    holder.CGST.setText(HSN.get(position).getCGST());


    }

    @Override
    public int getItemCount() {
        return Hsn.size();
    }

    static class MyViewHolder extends RecyclerView.ViewHolder {
        TextView GSTCODE,CODE;

        MyViewHolder(View itemView) {
            super(itemView);
            GSTCODE = itemView.findViewById(R.id.GSTCODE);
            CODE = itemView.findViewById(R.id.CODE);
         //   CGST = itemView.findViewById(R.id.CGST);
        }
    }

}

You are not creating object.

You should add this line in your onCreate method of your main activity:
ac = new Adapter(call, this);

And declare Adapter ac; globally.

Also add:
recyclerView.setAdapter(ac);
in onCreate method after initialising recyclerview layout

public class MainActivity extends AppCompatActivity {

    private RecyclerView recyclerView;
    private RecyclerView.LayoutManager layoutManager;
    private List<hsn> Hsn;
    private Adapter adapter;
    private ApiInterface apiInterface;
    ProgressBar progressBar;
    **Adapter ac;**


    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);


        **ac = new Adapter(call, this);**

        progressBar = findViewById(R.id.progress);
        recyclerView = findViewById(R.id.recycler);

                **recyclerView.setLayoutManager(new LinearLayoutManager(getContext(), RecyclerView.VERTICAL, false));**


        **recyclerView.setAdapter(ac);**





        fetchhsn(""); //without keyword

    }

    public void fetchhsn (String key){
        apiInterface = ApiClient.getApiClient().create(ApiInterface.class);
        Call<List<hsn>> call = apiInterface.gethsn(key);

//        adapter = new Adapter(Hsn, MainActivity.this);


        call.enqueue(new Callback<List<hsn>>() {
            @Override
            public void onResponse(Call<List<hsn>> call, Response<List<hsn>> response) {
                adapter = new Adapter(Hsn,MainActivity.this);
                progressBar.setVisibility(View.GONE);
                recyclerView.setAdapter(adapter);
                adapter.notifyDataSetChanged();
                Hsn = response.body();

            }

            @Override
            public void onFailure(Call<List<hsn>> call, Throwable t) {
                progressBar.setVisibility(View.GONE);
                Toast.makeText(MainActivity.this,"ERROR ON :" + t.toString(), Toast.LENGTH_LONG).show();

            }
        });
    }

    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        MenuInflater inflater = getMenuInflater();
        inflater.inflate(R.menu.menu, menu);

        SearchManager searchManager = (SearchManager) getSystemService(Context.SEARCH_SERVICE);
        SearchView searchView = (SearchView) menu.findItem(R.id.search).getActionView();

        searchView.setSearchableInfo(
                searchManager.getSearchableInfo(getComponentName())
        );

        searchView.setIconifiedByDefault(false);
        searchView.setOnQueryTextListener(new SearchView.OnQueryTextListener() {
            @Override
            public boolean onQueryTextSubmit(String query) {
                fetchhsn(query);
                return false;
            }

            @Override
            public boolean onQueryTextChange(String newText) {
                fetchhsn(newText);
                return false;
            }
        });

        return true;
    }
}

Try the edits I have mentioned in bold(between two asterics).