如何在改造时获得非空响应

Question

// Main_Activity.java

public class MainActivity extends AppCompatActivity implements View.OnClickListener {

    TextView t1;
    Button b1;
    String one,key;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        t1= (TextView) findViewById(R.id.textView);
        b1= (Button) findViewById(R.id.but_click);
        b1.setOnClickListener(this);
    }

    @Override
    public void onClick(View v) {


        peopleDetails();
    }

    public void peopleDetails()
    {

        Retrofit retrofit=new Retrofit.Builder()
                .baseUrl("http://echo.jsontest.com/key/value/one/")
                .addConverterFactory(GsonConverterFactory.create())
                .build();

        Apiservice service=retrofit.create(Apiservice.class);
        Call<Modelclass> call=service.peopleDetails();

        call.enqueue(new Callback<Modelclass>() {
            @Override
            public void onResponse(Call<Modelclass> call, Response<Modelclass> response) {
                Toast.makeText(MainActivity.this, "onresponse", Toast.LENGTH_LONG).show();
                key=response.body().getKey();

                t1.setText(key.toString());


            }

            @Override
            public void onFailure(Call<Modelclass> call, Throwable t) {
                Toast.makeText(MainActivity.this, "onfail", Toast.LENGTH_LONG).show();
            }
        });
    }
}

上面的代码抛出了nullpointerexeption。

我正在使用此网址请求：
   http://echo.jsontest.com/key/value/one/two