Using Kinvey v3.0.4.

UserStore.signUp(username, password, client, new KinveyUserCallback<User>() {
                @Override
                public void onSuccess(User user) {
                    client.getActiveUser().set("email", email);

                    client.getActiveUser().update(new KinveyClientCallback() {
                        @Override
                        public void onSuccess(Object o) {
                            
                        }

                        @Override
                        public void onFailure(Throwable throwable) {
                            Log.d("myTag", throwable.getMessage());
                        }
                    });
                   // callback.onSuccess(user);
                }

                @Override
                public void onFailure(Throwable throwable) {
                    callback.onFailure(throwable);
                }
            });

Signing up a user is successful but updating the user fails with the Message 

"currentUser must not be null". 

After signup:

client.getActiveUser().getUsername() != null

client.isUserLoggedIn() = true

What does the error mean?

Calling Client client = new Client.Builder(context).build(); more than once and in different classes will cause the error "currentUser must not be null".