Bitmap is null when being observed two times by Live Data

Question

I have ImageView imgProfil in MojProfilActivity and I want to load a picture from a database there. It works just fine the first time, but after I try it the second time the bmp is null for some strange reason. Can you help, please?

MutableLiveData<ResponseBody> mld = UserRepository.getInstance(MainApplication.apiManager).GetProfilna(Utils.getInstance().getKorisnik().getId());
            mld.observe(MojProfilActivity.this, new Observer<ResponseBody>() {
                @Override
                public void onChanged(ResponseBody responseBody) {
                    bmp= BitmapFactory.decodeStream(responseBody.byteStream());
                    imgProfil.setImageBitmap(bmp);
                }
            });

This is my interface:

public interface RestInterface {
    @GET("users/getprofilna/{UserId}")
    Call<ResponseBody> GetProfilna(@Path("UserId") String UserId);
}

This is my Api Manager

public class ApiManager {
    private static RestInterface service;
    private static ApiManager apiManager;

    private ApiManager(){
        service = RetrofitService.Create();
    }
    public static ApiManager getInstance(){
        if(apiManager == null) {
            apiManager = new ApiManager();
        }
        return  apiManager;
    }
    public void GetProfilna(String UserId, Callback<ResponseBody> callback){
        Call<ResponseBody> getProfilna = service.GetProfilna(UserId);
        getProfilna.enqueue(callback);
    }
}

This is my UserRespository class:

public class UserRepository {
    private static volatile UserRepository instance;
    private final ApiManager apiManager;

    private final MutableLiveData<ResponseBody> responseBody = new MutableLiveData<>();

    private UserRepository(ApiManager apiManager){
        this.apiManager = apiManager;
    }

    public static UserRepository getInstance(ApiManager apiManager){
        if(instance == null){
            instance = new UserRepository(apiManager);
        }
        return instance;
    }
    public MutableLiveData<ResponseBody> GetProfilna(String UserId){
        apiManager.GetProfilna(UserId, new Callback<ResponseBody>() {
            @Override
            public void onResponse(Call<ResponseBody> call, Response<ResponseBody> response) {
                if(response.isSuccessful()){
                    responseBody.setValue(response.body());
                }
            }

            @Override
            public void onFailure(Call<ResponseBody> call, Throwable t) {
            }
        });

        return responseBody;
    }
}