新手问下HttpClient出现空指针异常怎么解决

public class MainActivity extends AppCompatActivity implements View.OnClickListener {
    TextView responseview;
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.mylayout);
        Button button1 = (Button) findViewById(R.id.send_button);
        responseview = (TextView) findViewById(R.id.respose_text);
        button1.setOnClickListener(this);
    }    public void onClick(View v) {
        if (v.getId() == R.id.send_button) {
            sendRequestWithOkhttp();
        }
    }
    private void sendRequestWithOkhttp() {
        new Thread(new Runnable() {
            @Override
            public void run() {
                try {
                    OkHttpClient client = new OkHttpClient();
                    Request request = new Request.Builder().url("http://www.baidu.com").build();
                    Response response = client.newCall(request).execute();
                    String responsedata = response.body().string();
                    showResponse(responsedata);
                } catch (Exception e) {
                    e.printStackTrace();
                }
            }
        }).start();
    }
    private void showResponse(final String response) {
        runOnUiThread(new Runnable() {
            @Override
            public void run() {
                responseview.setText(response);
            }
        });
    }
}