问试图对空对象引用调用接口方法'java.lang.Object java.util.List.remove(int)‘|正在尝试从回收器视图中删除

EN

public class MainActivity extends AppCompatActivity {

    private Toolbar mainToolBar;

    private AdView mAdView;

    RecyclerView recyclerView;
    ProductAdapter adapter;

    List<Product> productList;


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




        loadData();



        recyclerView = (RecyclerView) findViewById(R.id.recyclerView);
        recyclerView.setHasFixedSize(true);

        recyclerView.setLayoutManager(new LinearLayoutManager(this));



        adapter = new ProductAdapter(this, productList);
        recyclerView.setAdapter(adapter);

    }

    @Override
    public boolean onCreateOptionsMenu(Menu menu) {

        getMenuInflater().inflate(R.menu.main_menu, menu);

        return true;
    }

    @Override
    public boolean onOptionsItemSelected(MenuItem item) {

        startActivity(new Intent(MainActivity.this, activity_add.class));


        return true;
    }



    public void deleteItem(int id){

        id = id - 1;
        productList.remove(id);
        adapter.notifyItemRemoved(id);
    }
    private void loadData(){
        SharedPreferences sharedPreferences = getSharedPreferences("shared 
        prefs", 0);
        Gson gson = new Gson();
        String json = sharedPreferences.getString("items", null);
        Type type = new TypeToken<ArrayList<Product>>(){}.getType();
        productList = gson.fromJson(json, type);

        if(productList == null){
            productList = new ArrayList<>();
        }
    }

    private void addToList(){
        SharedPreferences sharedPreferences = getSharedPreferences("shared prefs", 0);
        SharedPreferences.Editor editor = sharedPreferences.edit();
        Gson gson = new Gson();
        String json = gson.toJson(productList);
        editor.putString("items", json);
        editor.apply();
    }

}

public class ProductAdapter extends RecyclerView.Adapter<ProductAdapter.ProductViewHolder>{

    private Context mCtx;
    private List<Product> productList;

    public ProductAdapter(Context mCtx, List<Product> productList) {
        this.mCtx = mCtx;
        this.productList = productList;
    }

    @NonNull
    @Override
    public ProductViewHolder onCreateViewHolder(@NonNull ViewGroup parent, int viewType) {

        LayoutInflater inflater = LayoutInflater.from(mCtx);
        View view = inflater.inflate(R.layout.list_layout, null);
        return new ProductViewHolder(view);
    }

    @Override
    public void onBindViewHolder(@NonNull ProductViewHolder holder, int position) {
        final Product product = productList.get(position);

        holder.textViewTitle.setText(product.getTitle());
        holder.textViewRating.setText(String.valueOf(product.getRating()));



        holder.deleteBtn.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                new MainActivity().deleteItem(product.getId());
            }
        });
    }

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

    class ProductViewHolder extends RecyclerView.ViewHolder{

        TextView textViewTitle, textViewRating;

        Button deleteBtn;

        public ProductViewHolder(View itemView) {
            super(itemView);

            textViewTitle = itemView.findViewById(R.id.textViewTitle);
            textViewRating = itemView.findViewById(R.id.textViewRating);

            deleteBtn = itemView.findViewById(R.id.deleteBtn);

        }
    }
}