NotifyDatasetchanged not working. AppCrashes after ButtonOnClick

I've been trying to create a button that adds a new Value to my Arraylist (contries) when clicked, but it instead of adding my item(Card) the app simply crashes. I am also using Fragments. Here's my code.

That one is my Class for my FragmentPage

package com.example.barryblack;
import android.os.Bundle;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.Button;
import androidx.annotation.NonNull;
import androidx.annotation.Nullable;
import androidx.fragment.app.Fragment;
import androidx.recyclerview.widget.LinearLayoutManager;
import androidx.recyclerview.widget.RecyclerView;
import java.util.ArrayList;
public class FragmentPage extends Fragment {
private RecyclerView.Adapter mAdapter;
private ArrayList countries;
Button btn;
@Nullable
@Override
public View onCreateView(@NonNull LayoutInflater inflater, @Nullable ViewGroup container, @Nullable 
Bundle savedInstanceState) {
CreateExampleList();
View view = inflater.inflate(R.layout.page_fragment, container, false);
RecyclerView recyclerView = view.findViewById(R.id.listRecyclerView);
btn = view.findViewById(R.id.button_insert);
btn.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
InsertItem();
}
});
recyclerView.setHasFixedSize(true);
RecyclerView.LayoutManager layoutManager = new LinearLayoutManager(getActivity());
recyclerView.setLayoutManager(layoutManager);
RecyclerView.Adapter adapter = new MyAdapter(countries);
recyclerView.setAdapter(adapter);
return view;
}
public void InsertItem() {
countries.add("Smile");
mAdapter.notifyDataSetChanged();
}
private void CreateExampleList() {
countries = new ArrayList<String>();
countries.add("Eat");
countries.add("Kill");
}
}

This is my CustomAdapter

public class MyAdapter extends RecyclerView.Adapter<MyAdapter.ViewHolder> {
private ArrayList<String> cards;
public MyAdapter(ArrayList<String> countries) {
this.cards = countries;
}
@Override
public MyAdapter.ViewHolder onCreateViewHolder(ViewGroup viewGroup, int i) {
View view = LayoutInflater.from(viewGroup.getContext()).inflate(R.layout.example_item, viewGroup, 
false);
return new ViewHolder(view);
}
@Override
public void onBindViewHolder(MyAdapter.ViewHolder viewHolder, int i) {
viewHolder.tv_country.setText(cards.get(i));
}
@Override
public int getItemCount() {
return cards.size();
}
public class ViewHolder extends RecyclerView.ViewHolder{
private TextView tv_country;
private Button tv_button;
public ViewHolder(View view) {
super(view);
tv_country = (TextView)view.findViewById(R.id.tv_country);
}
}
}public class MyAdapter extends RecyclerView.Adapter<MyAdapter.ViewHolder> {
private ArrayList<String> cards;
public MyAdapter(ArrayList<String> countries) {
this.cards = countries;
}
@Override
public MyAdapter.ViewHolder onCreateViewHolder(ViewGroup viewGroup, int i) {
View view = LayoutInflater.from(viewGroup.getContext()).inflate(R.layout.example_item, viewGroup, 
false);
return new ViewHolder(view);
}
@Override
public void onBindViewHolder(MyAdapter.ViewHolder viewHolder, int i) {
viewHolder.tv_country.setText(cards.get(i));
}
@Override
public int getItemCount() {
return cards.size();
}
public class ViewHolder extends RecyclerView.ViewHolder{
private TextView tv_country;
private Button tv_button;
public ViewHolder(View view) {
super(view);
tv_country = (TextView)view.findViewById(R.id.tv_country);
}
}
}

And my LogcatText

java.lang.NullPointerException: Attempt to invoke virtual method 'void androidx.recyclerview.widget.RecyclerView$Adapter.notifyDataSetChanged()' on a null object reference
at com.example.barryblack.FragmentPage.InsertItem(FragmentPage.java:60)
at com.example.barryblack.FragmentPage$1.onClick(FragmentPage.java:43)

this code private RecyclerView.Adapter mAdapter;

must be private MyAdapter mAdapter;

and this RecyclerView.Adapter adapter = new MyAdapter(countries);

to this MyAdapter mAdapter = new MyAdapter(countries);