致命异常：异步任务3

Question

当我尝试运行我的应用时出现以下错误代码：

05-06 19:47:05.977 19671-19705/com.example.matth_000.weather E/AndroidRuntime: FATAL EXCEPTION: AsyncTask #3
   Process: com.example.matth_000.weather, PID: 19671
   java.lang.RuntimeException: An error occured while executing doInBackground()
       at android.os.AsyncTask$3.done(AsyncTask.java:304)
       at java.util.concurrent.FutureTask.finishCompletion(FutureTask.java:355)
       at java.util.concurrent.FutureTask.setException(FutureTask.java:222)
       at java.util.concurrent.FutureTask.run(FutureTask.java:242)
       at android.os.AsyncTask$SerialExecutor$1.run(AsyncTask.java:231)
       at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1112)
       at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:587)
       at java.lang.Thread.run(Thread.java:818)
    Caused by: java.lang.RuntimeException: Can't create handler inside thread that has not called Looper.prepare()
       at android.os.Handler.<init>(Handler.java:200)
       at android.os.Handler.<init>(Handler.java:114)
       at android.widget.Toast$TN.<init>(Toast.java:344)
       at android.widget.Toast.<init>(Toast.java:100)
       at android.widget.Toast.makeText(Toast.java:258)
       at com.example.matth_000.weather.MainActivity$DownloadTask.doInBackground(MainActivity.java:95)
       at com.example.matth_000.weather.MainActivity$DownloadTask.doInBackground(MainActivity.java:62)
       at android.os.AsyncTask$2.call(AsyncTask.java:292)
       at java.util.concurrent.FutureTask.run(FutureTask.java:237)
       at android.os.AsyncTask$SerialExecutor$1.run(AsyncTask.java:231) 
       at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1112) 
       at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:587) 
       at java.lang.Thread.run(Thread.java:818) 

以下是我的MainActivity.java文件的副本：

public class MainActivity extends AppCompatActivity {

    EditText cityName;
    TextView resultTextView;

    public void findWeather(View view){

        Log.i("cityName", cityName.getText().toString());

        InputMethodManager mgr = (InputMethodManager) getSystemService(Context.INPUT_METHOD_SERVICE);
        mgr.hideSoftInputFromWindow(cityName.getWindowToken(), 0);

        try {
            String encodedCityName = URLEncoder.encode(cityName.getText().toString(), "UTF-8");

            DownloadTask task = new DownloadTask();
            task.execute("http://api.openweathermap.org/data/2.5/weather?q=" + encodedCityName + "3aa15a7cf936d3aa5bdb22c57103f89a");

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

            Toast.makeText(getApplicationContext(), "Could not find weather", Toast.LENGTH_LONG);
        }
    }

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

        cityName = (EditText)findViewById(R.id.cityName);
        resultTextView = (TextView)findViewById(R.id.resultTextView);
    }

    public class DownloadTask extends AsyncTask<String, Void, String> {
        @Override
        protected String doInBackground(String... urls) {
            String result = "";
            URL url;
            HttpURLConnection urlConnection = null;

            try {
                url = new URL(urls[0]);

                urlConnection = (HttpURLConnection) url.openConnection();
                InputStream in = urlConnection.getInputStream();
                InputStreamReader reader = new InputStreamReader(in);
                int data = reader.read();

                while (data != 1) {
                    char current = (char) data;
                    result += current;
                    data = reader.read();
                }
                return result;
            } catch (Exception e) {
               Toast.makeText(getApplicationContext(), "Could not find weather", Toast.LENGTH_LONG);
            }
            return null;
        }

        @Override
        protected void onPostExecute(String result) {
            super.onPostExecute(result);
            try {
                String message = "";

                JSONObject jsonObject = new JSONObject(result);

                String weatherInfo = jsonObject.getString("weather");

                Log.i("Weather content", weatherInfo);

                JSONArray arr = new JSONArray(weatherInfo);

                for (int i = 0; i < arr.length(); i++) {

                    JSONObject jsonPart = arr.getJSONObject(i);

                    String main = "";
                    String description = "";

                    main = jsonPart.getString("main");
                    description = jsonPart.getString("description");

                    if (main != "" && description != "") {
                        message += main + ": " + description + "\r\n";

                    }
                }

                if (message != "") {
                    resultTextView.setText(message);
                } else {

                    Toast.makeText(getApplicationContext(), "Could not find weather", Toast.LENGTH_LONG);
                }

            } catch (JSONException e) {
                Toast.makeText(getApplicationContext(), "Could not find weather", Toast.LENGTH_LONG);
            }
        }
    }
}

我哪里错了？