解析JSONObject时出现NullPointer异常

Question

运行此代码时获取NullPointer异常：

public class MainActivity extends AppCompatActivity {

TextView textView;
String JSONstr;
String title;
JSONObject weatherInfo;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    textView = (TextView) findViewById(R.id.textView);

    weatherThread weather = new weatherThread();
    weather.execute();

    Log.d("LOL1","TESTING");

    try {
        weatherInfo=new JSONObject(JSONstr);
        title=weatherInfo.getJSONObject("query").getString("created");
    } catch (JSONException e) {
        e.printStackTrace();
    }


}
public class weatherThread extends AsyncTask<Void, Void, Void>{
    @Override
    protected Void doInBackground(Void... params) {
        URL url;
        URLConnection urlConnection;
        InputStream inputStream;
        BufferedReader bufferedReader;
        try {
            url=new URL("https://query.yahooapis.com/v1/public/yql?q=select%20*%20from%20weather.forecast%20where%20woeid%20in%20(select%20woeid%20from%20geo.places(1)%20where%20text%3D%2208852%22)&format=json&env=store%3A%2F%2Fdatatables.org%2Falltableswithkeys");
            urlConnection = url.openConnection();
            inputStream = urlConnection.getInputStream();
            bufferedReader = new BufferedReader(new InputStreamReader(inputStream));
            JSONstr=bufferedReader.readLine();
            bufferedReader.close();
            Log.d("TEST",JSONstr);
        } catch (IOException e) {Log.d("ERROR",e.toString());}
        return null;
    }

    @Override
    protected void onPostExecute(Void aVoid) {
    }

    @Override
    protected void onPreExecute() {
        super.onPreExecute();
    }
}

}

这是JSONObject的链接

    https://query.yahooapis.com/v1/public/yql?q=select%20*%20from%20weather.forecast%20where%20woeid%20in%20(select%20woeid%20from%20geo.places(1)%20where%20text%3D%2208852%22)&format=json&env=store%3A%2F%2Fdatatables.org%2Falltableswithkeys

我试着查看其他帖子，但他们的代码完全不同于我的。 try catch语句出现错误。

提前谢谢

编辑：为什么你把它标记为关于＆＃34;什么是空指针异常＆＃34;的问题的副本。这个问题是为什么它会在特定情况下出现。