Android Null Pointer Exception in Retrofit response

Question

In my application i have been using retrofit for my webservice calls. It is working fine but when application goes in to background it crashes and got the error log as,

  java.lang.NullPointerException: Attempt to invoke interface method 'void    retrofit.Callback.failure(retrofit.RetrofitError)' on a null object   reference
    at retrofit.CallbackRunnable$2.run(CallbackRunnable.java:53)
    at android.os.Handler.handleCallback(Handler.java:739)
    at android.os.Handler.dispatchMessage(Handler.java:95)
    at android.os.Looper.loop(Looper.java:145)
    at android.app.ActivityThread.main(ActivityThread.java:5942)
    at java.lang.reflect.Method.invoke(Native Method)
    at java.lang.reflect.Method.invoke(Method.java:372)
    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1400)
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1195)


Can anyone help me to overcome this issue..??

This is my Retrofit client class,

public class RetrofitClient {
private static RetrofitCommonService RETROFIT_CLIENT;
public RetrofitClient() {}
public static RetrofitCommonService getInstance() {
    //if REST_CLIENT is null then set-up again.
    if (RETROFIT_CLIENT == null) {
        setupRestClient();
    }
    return RETROFIT_CLIENT;
}

private static void setupRestClient() {
    RestAdapter restAdapter = new RestAdapter.Builder()
            .setClient(new OkClient(new OkHttpClient()))
            .setEndpoint(Constants.BASE_URL)
            //.setLogLevel(RestAdapter.LogLevel.FULL)
            .build();
    RETROFIT_CLIENT = restAdapter.create(RetrofitCommonService.class);
}
}

This is my Retrofit Service interface,

 public interface RetrofitCommonService {

@FormUrlEncoded
@POST("/user")
void doRegistration(@Field("user[mobile_number]") String phone, @Field("user[new_uid]") String imei,
                    Callback<RetrofitResponse> response);

 }

And this way i am making a call to Retrofit service from my activity and activity implementing retrofit callbacks.

 RetrofitCommonService mRetrofitCommonService = RetrofitClient.getInstance();
    mRetrofitCommonService.doRegistration(mPhoneNumber, getDeviceId(), this);

public class RegistrationActivity extends Activity implements Callback<RetrofitResponse>{

}

@Override
public void success(RetrofitResponse retrofitResponse, Response response) {

      }

@Override
public void failure(RetrofitError error) {
}  

Answer 1

Fixed this issue.

In my activity i have two Retrofit services operations and these two services relay on same Retrofit Callback methods. And these causes null pointer when one service try to get a callback at the same time callback is used by other service. I have removed the callback methods and used return objects from Retrofit service. Then relaying on the callback method is not needed.

This worked for me...