我想通過改造從 MySQL 數據庫中獲取最后一個 id 但是當運行項目有錯誤時，使用郵遞員測試的調用方法值并獲取值APIClient.java :public class APIClient {private static final String BASE_URL = "http://**********/Api/";private static Retrofit retrofit = null;public static Retrofit getClient() {    if (retrofit == null) {        retrofit = new Retrofit.Builder().baseUrl(BASE_URL).addConverterFactory(GsonConverterFactory.create())                .build();    }    return retrofit;}}APIIinterface.java :public interface APIIinterface {@POST("Agahi/getlastid.php")Call<AgahiLastId> getAgahiLastId(); }AgahiLastId.java :public class AgahiLastId {    @SerializedName("agahi_id")    private int agahi_id;    public int getAgahi_id() {        return agahi_id;    }    public void setAgahi_id(int agahi_id) {        this.agahi_id = agahi_id;    }}主活動.java :       public class MainActivity extends AppCompatActivity {private APIIinterface apiIinterface;@Overrideprotected void onCreate(Bundle savedInstanceState) {    super.onCreate(savedInstanceState);    setContentView(R.layout.activity_main);Call<AgahiLastId> agahiLastIdCall = apiIinterface.getAgahiLastId();agahiLastIdCall.enqueue(new Callback<AgahiLastId>() {    @Override    public void onResponse(Call<AgahiLastId> call, Response<AgahiLastId> response) {        if (response.isSuccessful()) {            Log.d("lts","ok");        }    }    @Override    public void onFailure(Call<AgahiLastId> call, Throwable t) {    }});}我有這一行的錯誤：Call<AgahiLastId> agahiLastIdCall = apiIinterface.getAgahiLastId();錯誤日志： Attempt to invoke the interface method 'retrofit2. Call app.zagroszoom.mseif.com.zagroszoom.webService.APIIinterface.getAgahiLastId()' on a null object reference