java.lang.NullPointerException: Attempt to invoke virtual method 'okhttp3.Call okhttp3.OkHttpClient.newCall(okhttp3.Request)' on a null object reference

지금 이런 에러가 뜨는데 get은 okhttp 를 통해 되는데 json 데이터를 post하는게 안되네요 ㅜㅜ 구글링해가면서 하는데 잘 안됩니다.

public class PostComments {

        String doPostRequest(String url, String json) throws IOException {

            RequestBody body = RequestBody.create(JSON, json);

            Log.e("tag", " asfasdfadfasdf " + json);

            Request request = new Request.Builder()
                    .header("Content-Type", "application/json")
                    .url(url)
                    .post(body)
                    .build();

            //Response response;

            client2.newCall(request).enqueue(new Callback() {

                @Override
                public void onFailure(Call call, IOException e) {

                }

                @Override
                public void onResponse(Call call, Response response) throws IOException {

                    final String res = response.body().string();

                    runOnUiThread(new Runnable() {
                        @Override
                        public void run() {
                            try {
                                JSONObject jsonObject = new JSONObject(res);
                                String parentJArray = jsonObject.getString("responseCode");

                            } catch (JSONException e) {
                                e.printStackTrace();
                            }


                        }
                    });
                }
            });


@Override
public void onClick(View v) {
    switch (v.getId()) {
        case R.id.writeCommentsBtn:
            JSONObject jsonObject = new JSONObject();

            if(content == null) {
                Toast.makeText(getApplicationContext(), "내용을 입력하세요", Toast.LENGTH_SHORT).show();
            } else {
                try {

                    jsonObject.put("board_id", boardId);
                    jsonObject.put("content", content);

                    pjson = jsonObject.toString();
                    Log.e("tag", boardId + "     " + content + "    "  +pjson);

                } catch (JSONException e) {
                    e.printStackTrace();
                }

                PostComments postComments = new PostComments();
                try {
                    postComments.doPostRequest("https://api.3rd.supply/comments", pjson);
                } catch (IOException e) {
                    e.printStackTrace();
                }

클릭 했을 때 통신을 하는데 저렇게 코딩을 했는데 왜 에러가 뜰까요? ㅜ null object 에러가 뜨는데 제가 데이터를 잘못보내고 있는건가요?