My recycle view problem

This is the return from the API using Postman:

[
    {
        "id": 0,
        "ponumber": "5222019000730"
    },
    {
        "id": 1,
        "ponumber": "5222019000731"
    }
]

This is the code I am using in the app top  retrieve the information:

    private void readOutStandingPO() {
        PoList.clear();
        managerReceivingApi.getOutStandingPO(new Callback<ArrayList<ObjectPo>>() {
            @Override
            public void onResponse(@NonNull Call<ArrayList<ObjectPo>> call, @NonNull Response<ArrayList<ObjectPo>> response) {
                ArrayList<ObjectPo> JsonList = response.body();
                for (AtomicInteger i = new AtomicInteger(); i.get() < Objects.requireNonNull(JsonList).size(); i.getAndIncrement())
                    PoList.add(new ObjectPo(
                            JsonList.get(i.get()).getId(),
                            JsonList.get(i.get()).getPoNum()
                    ));
                pAdapter.notifyDataSetChanged();
                Toast.makeText(getApplicationContext(),"Out Standing PO's Loaded", Toast.LENGTH_SHORT).show();
            }
            @Override
            public void onFailure(@NonNull Call<ArrayList<ObjectPo>> call, @NonNull Throwable t) {
                Toast.makeText(getApplicationContext(), t.getMessage(), Toast.LENGTH_SHORT).show();
            }
        });
    }

This is the adapter for the recycleview I am using to display the list:

 public class PoAdapter extends RecyclerView.Adapter<PoAdapter.PoViewHolder> {
        final private ArrayList<ObjectPo> mPoList;
        class  PoViewHolder extends RecyclerView.ViewHolder {
            final Button mPoButton;
            PoViewHolder(View PoView) {
                super(PoView);
                mPoButton = PoView.findViewById(R.id.btnPO);
            }
        }
        PoAdapter(ArrayList<ObjectPo> PoList) {
            mPoList = PoList;
        }
        @NonNull
        @Override
        public PoViewHolder onCreateViewHolder(@NonNull ViewGroup parent, int viewType) {
            View v = LayoutInflater.from(parent.getContext()).inflate(R.layout.list_po, parent, false);
            return new PoViewHolder(v);
        }
        @Override
        public void onBindViewHolder(@NonNull PoViewHolder holder, int position) {
            final ObjectPo currentPo = mPoList.get(position);
            holder.mPoButton.setText(currentPo.getPoNum());
            holder.mPoButton.setOnClickListener(new View.OnClickListener() {
                @Override
                public void onClick(View view) {

                }
            });
        }

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

If you need more information please let me know.