尝试在空对象引用上调用虚方法'java.lang.String org.ksoap2.serialization.SoapPrimitive.toString（）'

Question

编辑：
我知道它返回空指针，但是它的原因是，它是某些Web服务还是代码问题？

我正在从Android调用外部WebService（celcius到Farenheit Demo）。

它给了我以下错误

Attempt to invoke virtual method 'java.lang.String org.ksoap2.serialization.SoapPrimitive.toString()' on a null object reference

我没有太多的代码，只有MainActivity类和异步类

public class MainActivity extends AppCompatActivity {

String TAG = "Response";
Button bt;
EditText celcius;
String getCel;
SoapPrimitive resultString;
protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        bt = (Button) findViewById(R.id.bt);
        celcius = (EditText) findViewById(R.id.cel);


        bt.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {

                getCel = celcius.getText().toString();
                AsyncCallWS task = new AsyncCallWS();
                task.execute();
            }
        });
    }

    private class AsyncCallWS extends AsyncTask<Void, Void, Void> {

        @Override
        protected void onPreExecute() {
            Log.i(TAG, "onPreExecute");
        }

        @Override
        protected Void doInBackground(Void... params) {
                Log.i(TAG, "doInBackground");
            calculate();
            return null;
        }

        public void calculate() {
            String SOAP_ACTION = "http://www.w3schools.com/webservices/CelsiusToFahrenheit";
            String METHOD_NAME = "CelsiusToFahrenheit";
            String NAMESPACE = "http://www.w3schools.com/webservices/";
            String URL = "http://www.w3schools.com/webservices/tempconvert.asmx";

            try {
                SoapObject Request = new SoapObject(NAMESPACE, METHOD_NAME);
                Request.addProperty("Celsius", getCel);

                SoapSerializationEnvelope soapEnvelope = new SoapSerializationEnvelope(SoapEnvelope.VER11);
                soapEnvelope.dotNet = true;
                soapEnvelope.setOutputSoapObject(Request);

                HttpTransportSE transport = new HttpTransportSE(URL);

                transport.call(SOAP_ACTION, soapEnvelope);
                resultString = (SoapPrimitive) soapEnvelope.getResponse();

                Log.i(TAG, "Result Celsius: " + resultString);
            } catch (Exception ex) {
                Log.e(TAG, "Error: " + ex.getMessage());
            }
        }

        @Override
        protected void onPostExecute(Void result) {
            Log.i(TAG, "onPostExecute");
            Toast.makeText(MainActivity.this, "Response" + resultString.toString(), Toast.LENGTH_LONG).show();
        }

    }

Logcat显示

Attempt to invoke virtual method 'java.lang.String     
org.ksoap2.serialization.SoapPrimitive.toString()' on a null object     
reference at testproject.example.com.sensor_new.MainActivity$AsyncCallWS.onPostExecute(MainActivity.java:89)

我看到一些解决方案在清单中应用Internet设置并在DoBackGround中进行计算。但这并没有解决任何问题，

任何帮助将不胜感激