to invoke interface method 'retrofit2.Call on a null object reference - retrofit2

Question

how can i solve null object while sending post request
error says
to invoke interface method 'retrofit2.Call com.itgrepnet.foodbundle.remote.UserService.addUser(com.itgrepnet.foodbundle.model.User)' on a null object reference
in AddUserActivity
btnSubmit.setOnClickListener(new View.OnClickListener() {
#Override
public void onClick(View view) {
addNewUser();
}
private void addNewUser() {
User u = new User();
u.setFirstname(first_name.getText().toString());
u.setLastname(last_name.getText().toString());
u.setEmail(email.getText().toString());
u.setPassword(password.getText().toString());
Call<User> call = userService.addUser(u);
call.enqueue(new Callback<User>() {
#Override
public void onResponse(Call<User> call, Response<User> response) {
if (response.isSuccessful()) {
Toast.makeText(getApplicationContext(), "User Created Successfully!", Toast.LENGTH_LONG).show();
}
}
#Override
public void onFailure(Call<User> call, Throwable t) {
Log.e("Error: ", t.getMessage());
}
});
}
});
UserService.java
#POST("user/")
Call<User> addUser(#Body User user);
RetrofitClient.java
public class RetrofitClient {
private static Retrofit retrofit = null;
public static Retrofit getClient(String url) {
if (retrofit == null) {
retrofit = new Retrofit.Builder().baseUrl(url)
.addConverterFactory(GsonConverterFactory.create())
.build();
}
return retrofit;
}
}