How do I fix "E/RecyclerView: No adapter attached; skipping layout"?

I have a problem the code gives this error and, as a result, does not draw anything on the screen:

E/RecyclerView: No adapter attached; skipping layout

Here is my code:

package com.nikita.mozhaev.partyup.messaging;

import androidx.annotation.NonNull;
import androidx.appcompat.app.AppCompatActivity;
import androidx.recyclerview.widget.LinearLayoutManager;
import androidx.recyclerview.widget.RecyclerView;

import android.os.Bundle;
import android.view.View;

import com.google.firebase.auth.FirebaseAuth;
import com.google.firebase.auth.FirebaseUser;
import com.google.firebase.database.DataSnapshot;
import com.google.firebase.database.DatabaseError;
import com.google.firebase.database.DatabaseReference;
import com.google.firebase.database.FirebaseDatabase;
import com.google.firebase.database.ValueEventListener;
import com.nikita.mozhaev.partyup.R;
import com.nikita.mozhaev.partyup.system.Adapter.UserAdapter;
import com.nikita.mozhaev.partyup.system.Model.User;

import java.util.ArrayList;
import java.util.List;

public class DisplayUsers extends AppCompatActivity {

    private RecyclerView recyclerView;

    private UserAdapter userAdapter;
    private List<User> mUser;


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

        recyclerView = findViewById(R.id.users);
        recyclerView.setHasFixedSize(true);
        recyclerView.setLayoutManager(new LinearLayoutManager(getApplicationContext()));
        recyclerView.setAdapter(userAdapter);


        mUser = new ArrayList<>();

        readUsers();

    }

    private void readUsers() {

        FirebaseUser firebaseUser = FirebaseAuth.getInstance().getCurrentUser();
        DatabaseReference reference = FirebaseDatabase.getInstance().getReference("Users");

        reference.addValueEventListener(new ValueEventListener() {
            @Override
            public void onDataChange(@NonNull DataSnapshot snapshot) {
                mUser.clear();
                for (DataSnapshot datasnapshot : snapshot.getChildren()){
                    User user = datasnapshot.getValue(User.class);

                    assert firebaseUser != null;
                    assert user != null;
                    if (!user.getId().equals(firebaseUser.getUid())){
                        mUser.add(user);
                    }
                }

                userAdapter = new UserAdapter(getBaseContext(), mUser);
                recyclerView.setAdapter(userAdapter);
            }

            @Override
            public void onCancelled(@NonNull DatabaseError error) {

            }
        });
    }
}