RecyclerView isn't visible on Fragment

I'm following this tutorial to implement a Room database. Everything is the same except mine is on a Fragment. There are no errors and the RecyclerView cards aren't visible at all. (I'm still able to add cards dynamically when not using a RecyclerView)

I've already tried this on an Activity and it still doesn't work so I must have missed something as I'm not copying the entire code from the tutorial. Maybe it's something simple because I'm new to Android.

Adapter:

public class CartItemAdapter extends RecyclerView.Adapter<CartItemAdapter.CartItemHolder> {
    private List<CartItem> cartItems = new ArrayList<>();

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

    @Override
    public void onBindViewHolder(@NonNull CartItemHolder holder, int position) {
        CartItem currentCartItem = cartItems.get(position);
        holder.tvItemName.setText(currentCartItem.getItemName());
        holder.tvItemPrice.setText(currentCartItem.getItemPrice());
        holder.tvQuantity.setText(String.valueOf(currentCartItem.getQuantity()));
    }

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

    public void setCartItems(List<CartItem> cartItems) {
        this.cartItems = cartItems;
        notifyDataSetChanged();
    }

    class CartItemHolder extends RecyclerView.ViewHolder {
        private TextView tvItemName;
        private TextView tvItemPrice;
        private TextView tvQuantity;

        public CartItemHolder(@NonNull View itemView) {
            super(itemView);
            tvItemName = itemView.findViewById(R.id.itemName);
            tvItemPrice = itemView.findViewById(R.id.itemPrice);
            tvQuantity = itemView.findViewById(R.id.quantity);
        }
    }

}

ViewModel:

public class CartItemViewModel extends AndroidViewModel {
    private CartItemRepository repository;
    private LiveData<List<CartItem>> allCartItems;

    public CartItemViewModel(@NonNull Application application) {
        super(application);
        repository = new CartItemRepository(application);
        allCartItems = repository.getAllCartItems();
    }

    public void insert(CartItem cartItem) {
        repository.insert(cartItem);
    }

    public void update(CartItem cartItem) {
        repository.update(cartItem);
    }

    public void delete(CartItem cartItem) {
        repository.delete(cartItem);
    }

    public void deleteAllCartItems() {
        repository.deleteAll();
    }

    public LiveData<List<CartItem>> getAllCartItems() {
        return allCartItems;
    }
}

Fragment layout:

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:animateLayoutChanges="true"
    android:orientation="vertical">

    <!--Views...-->

    <androidx.recyclerview.widget.RecyclerView
        android:id="@+id/cart_recyclerview"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_above="@id/totalCard"
        android:layout_below="@id/titleCard"
        android:padding="8dp"
        tools:listitem="@layout/sample_product_item">

        <!-- cart items dynamically added here -->

    </androidx.recyclerview.widget.RecyclerView>

    <!--Views...-->

</RelativeLayout>

Fragment class:

    public View onCreateView(@NonNull LayoutInflater inflater, @Nullable ViewGroup container, @Nullable Bundle savedInstanceState) {
    view = inflater.inflate(R.layout.frag_cart, container, false);
    RecyclerView recyclerView = view.findViewById(R.id.cart_recyclerview);
        recyclerView.setLayoutManager(new LinearLayoutManager(getActivity()));
        recyclerView.setHasFixedSize(true);
        final CartItemAdapter adapter = new CartItemAdapter();
        recyclerView.setAdapter(adapter);

        cartItemViewModel = new ViewModelProvider(getActivity(),
                ViewModelProvider.AndroidViewModelFactory.getInstance(getActivity().getApplication())).get(CartItemViewModel.class);
        cartItemViewModel.getAllCartItems().observe(getActivity(), new Observer<List<CartItem>>() {
            @Override
            public void onChanged(List<CartItem> cartItems) {
                adapter.setCartItems(cartItems);
                Log.i(TAG, "items set!");
            }
        });
    return view;
}

I've populated the database in CartItemDatabase

@Database(entities = {CartItem.class}, version = 1)
public abstract class CartItemDatabase extends RoomDatabase {

    private static CartItemDatabase instance;

    public abstract CartItemDao cartItemDao();

    public static synchronized CartItemDatabase getInstance(Context context) {
        if (instance == null) {
            instance = Room.databaseBuilder(context.getApplicationContext(),
                    CartItemDatabase.class, "cart_item_database")
                    .fallbackToDestructiveMigration()
                    .addCallback(roomCallback)
                    .build();
        }
        return instance;
    }

    private static RoomDatabase.Callback roomCallback = new RoomDatabase.Callback() {
        @Override
        public void onCreate(@NonNull SupportSQLiteDatabase db) {
            super.onCreate(db);
            new PopulateDbAsyncTask(instance).execute();
        }
    };

    private static class PopulateDbAsyncTask extends AsyncTask<Void, Void, Void> {
        private CartItemDao cartItemDao;

        private PopulateDbAsyncTask(CartItemDatabase db) {
            cartItemDao = db.cartItemDao();
        }

        @Override
        protected Void doInBackground(Void... voids) {
            cartItemDao.insert(new CartItem("Bread", "60", 2));
            cartItemDao.insert(new CartItem("Butter", "70", 1));
            return null;
        }
    }

}

What am I doing wrong?
Please ask me if more info is needed. Thanks in advance.

Suggestion

Learn to debug your code and pinpoint your error. Most of the errors I get I solve them by debugging. If you don't know what debugging is or how to do it you can google it. It's really simple and it can save you hours of problem solving and it saves you the question altogether.

I'm suggesting this because from what you've described, the first thing I'd check is that if my RecyclerView is receiving anything at all. I would do this by setting a debug breakpoint inside the getItemCount method of my RecyclerView. If the value returned is bigger than 0 it means that's OK.

Then I'd proceed to check other steps of my code and this way you can actually see what's wrong and it's very useful to get a better understanding of what's going and how things work.

This way you can pinpoint your error and ask a more specific question. Instead of quoting your entire code, you could quote just a method or just one line.

EDIT

Great to hear that the problem is within the list. Now I see that you're having a problem with it in the first lines of your adapter. You're creating a new empty list when you do

private List<CartItem> cartItems = new ArrayList<>();

Maybe you can consider adding a parameter to the adapter such as

public class CartItemAdapter(List<CartItem> cartItems) extends RecyclerView.Adapter<CartItemAdapter.CartItemHolder>

And the you can use that list inside the adapter. It should be already populated. And in your onCreateView method you could do something like

// create the list
private List<CartItem> cartItems = new ArrayList<>();

// populate the list with some element or elements
CartItem item = new CartItem(constructor parameters);
cartItems.add(item);

// create the adapter and send the list as a parameter
final CartItemAdapter adapter = new CartItemAdapter(cartItems)
recyclerView.setAdapter(adapter)

And then in your adapter you can use cartItems as you wish