어플 실행이 자꾸 중지됩니다

Question

public class MainActivity extends AppCompatActivity implements View.OnClickListener {
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        Button btn = (Button)findViewById(R.id.button_call);
        btn.setOnClickListener(this);
    }
    @Override
    public void onClick(View v){
        Toast.makeText(getApplicationContext(),"미세먼지 조회", Toast.LENGTH_SHORT).show();
        EditText et_webpage_src = (EditText) findViewById(R.id.webpage_src);
        URL url ;
        HttpURLConnection urlConnection = null;
        BufferedInputStream buf = null;
        try{
            url= new URL("http://openapi.airkorea.or.kr/openapi/services/rest/ArpltnInforInqireSvc/getCtprvnMesureSidoLIst?serviceKey=nlioVxDR662tsiIWuR5YKgacitpj6B%2FEqqFGzdDhqrd%2FIHv5wTLOSMGnLRh2UKeP3NMkH5Os5r9gg5ry5sTj1g%3D%3D&numOfRows=10&pageNo=1&sidoName=%EC%84%9C%EC%9A%B8&_returnType=json&searchCondition=DAILY");
            urlConnection = (HttpURLConnection) url.openConnection();
            BufferedReader bufreader = new BufferedReader(new InputStreamReader(urlConnection.getInputStream(),"UTF-8"));
            Log.d("line:",bufreader.toString());
            String line ;
            String page = "";
            while((line = bufreader.readLine())!=null){
                Log.d("line:",line);
                page +=line;
            }
            JSONObject json = new JSONObject(page);
            JSONArray jArr = json.getJSONArray("item");
            for (int i=0; i<jArr.length(); i++){
                json = jArr.getJSONObject(i);
                String cityName = json.getString("cityName");
                String coValue = json.getString("coValue");
                String dataTime = json.getString("dataTime");
                String no2Value = json.getString("no2Value");
                String o3Value = json.getString("o3Value");
                String pm10Value = json.getString("pm10Value");
                String pm25Value = json.getString("pm25Value");
                String so2Value = json.getString("so2Value");
                System.out.println("cityName:"+cityName+"/coValue:"+coValue+"/dataTime"
+dataTime+"/no2Value"+no2Value+"/o3Value"+o3Value+"/pm10Value"+pm10Value+"/pm25Value"+pm10Value+"so2Value"+so2Value);
                et_webpage_src.append("[ "+cityName+" ]\n");
                et_webpage_src.append("[ "+coValue+" ]\n");
                et_webpage_src.append("[ "+dataTime+" ]\n");
                et_webpage_src.append("[ "+no2Value+" ]\n");
                et_webpage_src.append("[ "+o3Value+" ]\n");
                et_webpage_src.append("[ "+pm10Value+" ]\n");
                et_webpage_src.append("[ "+pm25Value+" ]\n");
                et_webpage_src.append("[ "+so2Value+" ]\n");
            }
        } catch (Exception e) {
            et_webpage_src.setText(e.getMessage());
        }finally{

             urlConnection.disconnect();
        }
        try {
            System.out.printf("i = %s \n");
        } catch (NullPointerException npe) {

            System.out.println(" Null Opint Exception!!");
        }
    }
}