I am using the developer.android.com/jetpack/docs/guide(arch components).I accross a problem with the remote backend(Retrofit)! How to work Retrofit and Room?

I have already implement the part for the loacal backend(Room) and the API class.Loading the data is successful.

public interface UserService { @FormUrlEncoded @POST("insert.php") Call<User> createUser(@Field("name") String nom, @Field("username") String username, @Field("email") String email, @Field("mdp") String mdp, @Field("role") String role); @GET("login.php") Call<User> getUserRemote(@Query("email") String email, @Query("mdp") String mdp); }

public class UserRepository { private UserDao userDao; private LiveData<List<User>> allusers; LiveData<User> user; final MutableLiveData<User> data = new MutableLiveData<>(); private UserService service;

public UserRepository(Application app) {
    Database_Project database= Database_Project.getInstance(app);
    userDao=database.userDao();
    allusers=userDao.getUsers();
    service=Webprovide.getClient().create(UserService.class);

}
public void insert(User user)
{
    new Insert_Asyntask(userDao).execute(user);
}
public void insertS(String name, String username,String email, String mdp, String role)
{
    User user=new User(nom,username,email,mdp,role);
    user.setNom(nom);
    user.setUsername(username);
    user.setRole(role);
    user.setEmail(email);
    user.setMdp(mdp);
    Call<User> call=service.createUser(user.getNom(),user.getUsername(),user.getEmail(),user.getMdp(),user.getRole());
    Log.d("user", "insertS: "+call.request());
    call.enqueue(new Callback<User>() {
        @Override
        public void onResponse(Call<User> call, Response<User> response) {
            Log.d("user", "insertS: "+call);
            Log.d("Response", "onResponse: "+response.body());
            data.setValue(response.body());
            Log.d("role", "insertS: "+response.body().getRole());

        }

        @Override
        public void onFailure(Call<User> call, Throwable t) {
            Log.d("Onfaillue", "onFailure: "+t.getLocalizedMessage() );
            Log.d("getmessage", "onFailure: "+t.getMessage());
            Log.d("Userservieces", "onFailure: "+service.toString());
            Log.d("on", "onFailure: ");
        }
    });
}
public void update(User user)
{
    new Update_Asyntask(userDao).execute(user);
}
public void delete(User user)
{
    new Delete_Asyntask(userDao).execute(user);
}
public LiveData<User> findMaterialsBorrowedByUser(int fk)
{
    return userDao.findMaterialsBorrowedByUser(fk);
}
public LiveData<List<User>> getAllusers(){

    return allusers;
}
public LiveData<User> getUser( final String email,final String mdp)  {

    Call<User> callUser=service.getUserRemote(email,mdp);
    callUser.enqueue(new Callback<User>() {
        @Override
        public void onResponse(Call<User> call, Response<User> response) {
            Log.d(TAG, "onResponse: "+response.code());
            if(response.isSuccessful())
            {
                data.setValue(response.body());
                user=data;
            }

        }

        @Override
        public void onFailure(Call<User> call, Throwable t) {
            Log.d(TAG, "onFailure: "+t.getMessage());
            data.setValue(null);
        }
    });
    user=userDao.getUser(email,mdp);
    return user;
}

private static class Insert_Asyntask extends AsyncTask<User, Void, Void>
{
    public Insert_Asyntask(UserDao userdao) {
        this.userdao = userdao;
    }

    private UserDao userdao;

    @Override
    protected Void doInBackground(User... users) {
        userdao.insert(users[0]);
        return null;
    }
}
private static class Update_Asyntask extends AsyncTask<User, Void, Void>
{
    public Update_Asyntask(UserDao userdao) {
        this.userdao = userdao;
    }

    private UserDao userdao;

    @Override
    protected Void doInBackground(User... users) {
        userdao.update(users[0]);
        return null;
    }
}
private static class Delete_Asyntask extends AsyncTask<User, Void, Void>
{
    public Delete_Asyntask(UserDao userdao) {
        this.userdao = userdao;
    }

    private UserDao userdao;

    @Override
    protected Void doInBackground(User... users) {
        userdao.delete(users[0]);
        return null;
    }


}

}

i have this error from the logcat: Attempt to invoke virtual method 'void android.arch.lifecycle.LiveData.observe(android.arch.lifecycle.LifecycleOwner, android.arch.lifecycle.Observer)' on a null object reference.

I espect to have retrieve the data from the remote backend and refresh the local backend!

Thanks!