java.lang.NullPointerException RecycyclerView

[shriabhishekmishra]

ERROR
java.lang.NullPointerException: Attempt to invoke virtual method 'void androidx.recyclerview.widget.RecyclerView.setHasFixedSize(boolean)' on a null object reference

`package com.computerscienceknowledge.zaaruri;

import android.content.Intent;
import android.os.Bundle;
import android.view.LayoutInflater;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;
import android.view.ViewGroup;
import android.widget.TextView;

import com.computerscienceknowledge.zaaruri.Model.Products;
import com.computerscienceknowledge.zaaruri.Prevalent.Prevalent;
import com.computerscienceknowledge.zaaruri.ViewHolder.ProductViewHolder;
import com.firebase.ui.database.FirebaseRecyclerAdapter;
import com.firebase.ui.database.FirebaseRecyclerOptions;
import com.google.android.material.floatingactionbutton.FloatingActionButton;
import com.google.android.material.navigation.NavigationView;
import com.google.android.material.snackbar.Snackbar;
import com.google.firebase.database.DatabaseReference;
import com.google.firebase.database.FirebaseDatabase;
import com.squareup.picasso.Picasso;

import androidx.annotation.NonNull;
import androidx.appcompat.app.ActionBarDrawerToggle;
import androidx.appcompat.app.AppCompatActivity;
import androidx.appcompat.widget.Toolbar;
import androidx.core.view.GravityCompat;
import androidx.drawerlayout.widget.DrawerLayout;
import androidx.recyclerview.widget.LinearLayoutManager;
import androidx.recyclerview.widget.RecyclerView;
//import androidx.recyclerview.widget.RecyclerView;
import de.hdodenhof.circleimageview.CircleImageView;
import io.paperdb.Paper;

public class HomeActivity extends AppCompatActivity
implements NavigationView.OnNavigationItemSelectedListener
{
DrawerLayout drawerLayout;
ActionBarDrawerToggle toggle;
Toolbar toolbar;
NavigationView navigationView;
View headerView;
TextView userNameTextView;
CircleImageView profileImageView;
ProductViewHolder holder;
View view;
private DatabaseReference ProductsRef;
private RecyclerView recyclerView;
RecyclerView.LayoutManager layoutManager;

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


    ProductsRef = FirebaseDatabase.getInstance().getReference().child("Products");

    Paper.init(this);


    toolbar = findViewById(R.id.toolBar);
    toolbar.setTitle("Home");
    setSupportActionBar(toolbar);


    FloatingActionButton fab = findViewById(R.id.fab);
    fab.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View view) {
            Snackbar.make(view, "Replace with your own action", Snackbar.LENGTH_LONG)
                    .setAction("Action", null).show();
        }
    });


    drawerLayout =  findViewById(R.id.drawer);
    toggle = new ActionBarDrawerToggle(this, drawerLayout,toolbar,R.string.open,R.string.close);
    drawerLayout.addDrawerListener(toggle);
    toggle.syncState();


    navigationView = findViewById(R.id.nav_view);
    navigationView.setNavigationItemSelectedListener(this);

    headerView = navigationView.getHeaderView(0);
    userNameTextView = headerView.findViewById(R.id.user_profile_name);
    profileImageView = headerView.findViewById(R.id.user_profile_image);
    userNameTextView.setText(Prevalent.currentOnlineUser.getName());

    //   Picasso.get().load(Prevalent.currentOnlineUser.getImage()).placeholder(R.drawable.profile).into(profileImageView);

    recyclerView = findViewById(R.id.recycler_menu);
    recyclerView.setHasFixedSize(true);
    layoutManager = new LinearLayoutManager(this);
    recyclerView.setLayoutManager(layoutManager);
}


@Override
protected void onStart()
{
    super.onStart();

    FirebaseRecyclerOptions<Products> options =
            new FirebaseRecyclerOptions.Builder<Products>()
                    .setQuery(ProductsRef, Products.class)
                    .build();

    FirebaseRecyclerAdapter<Products,ProductViewHolder> adapter =
            new FirebaseRecyclerAdapter<Products, ProductViewHolder>(options) {
                @Override
                protected void onBindViewHolder(@NonNull ProductViewHolder holder, int position, @NonNull Products model)
                {
                    holder.txtProductName.setText(model.getPname());
                    holder.txtProductDescription.setText(model.getDescription());
                    holder.txtProductPrice.setText("Price = " + model.getPrice() + "₹");
                    Picasso.get().load(model.getImage()).into(holder.imageView);
                }

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

    recyclerView.setAdapter(adapter);
    adapter.startListening();
}

@Override
public void onBackPressed() {
    drawerLayout = findViewById(R.id.drawer);
    if (drawerLayout.isDrawerOpen(GravityCompat.START)) {
        drawerLayout.closeDrawer(GravityCompat.START);
    } else {
        super.onBackPressed();
    }
}



@Override
public boolean onCreateOptionsMenu(Menu menu) {
    // Inflate the menu; this adds items to the action bar if it is present.
     getMenuInflater().inflate(R.menu.home, menu);
    return true;
}



@Override
public boolean onOptionsItemSelected(MenuItem item)
{
    int id = item.getItemId();

/*
if (id == R.id.action_settings)
{
return true;
}
*/
return super.onOptionsItemSelected(item);
}

@SuppressWarnings("StatementWithEmptyBody")
@Override
public boolean onNavigationItemSelected(MenuItem item)
{
    // Handle navigation view item clicks here.
    int id = item.getItemId();

    if (id == R.id.nav_cart)
    {

    }
    else if (id == R.id.nav_orders)
    {

    }
    else if (id == R.id.nav_categories)
    {

    }
    else if (id == R.id.nav_settings)
    {
        //  Intent intent = new Intent(HomeActivity.this, SettinsActivity.class);
        // startActivity(intent);
    }
    else if (id == R.id.nav_logout)
    {
        Paper.book().destroy();

        Intent intent = new Intent(HomeActivity.this, MainActivity.class);
        intent.addFlags(Intent.FLAG_ACTIVITY_NEW_TASK | Intent.FLAG_ACTIVITY_CLEAR_TASK);
        startActivity(intent);
        finish();
    }

    DrawerLayout drawer =  findViewById(R.id.drawer);
    drawer.closeDrawer(GravityCompat.START);
    return true;
}

}
`

content_home.xml

`

<androidx.recyclerview.widget.RecyclerView
    android:id="@+id/recycler_menu"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:scrollbars="vertical">

</androidx.recyclerview.widget.RecyclerView>

`