public static User createUser(String name, String lastName, String userName, String password, String email) {
    int user_id;
    user_id = realm.where(User.class).max("user_id").intValue() + 1; //Aqui me indica el error.....
    realm.beginTransaction();
    User newUser = realm.createObject(User.class, user_id);
    newUser.setName(name);
    newUser.setLast_name(lastName);
    newUser.setUsername(userName);
    newUser.setPassword(password);
    newUser.setEmail(email);
    realm.commitTransaction();
    return newUser;
}

public void saveData(View view){
        Context context = getApplicationContext();
        String names = name.getText().toString();
        String lastName = last_name.getText().toString();
        String userName = username.getText().toString();
        String passwordFirst = password.getText().toString();
        String confirmPassword = confirm_password.getText().toString();
        String emailFirst = email.getText().toString();
        String confirmEmail = confirm_email.getText().toString();
        char getValuePassword;
        byte countNumber = 0, countLetterMay = 0, countLetterMin = 0, countSimbol = 0;

        for (int i = 0; i < passwordFirst.length(); i++) {
            getValuePassword = passwordFirst.charAt(i);
            String passValue = String.valueOf(getValuePassword);
            if (passValue.matches("[A-Z]")) {
                countLetterMay++;
            } else if (passValue.matches("[a-z]")) {
                countLetterMin++;
            } else if (passValue.matches("[0-9]")) {
                countNumber++;
            } else if (!passValue.matches("[a-z]") && !passValue.matches("[A-Z]") && !passValue.matches("[0-9]")) {
                countSimbol++;
            }
        }
        if(names.isEmpty() || lastName.isEmpty() || userName.isEmpty() || passwordFirst.isEmpty() || confirmPassword.isEmpty() || emailFirst.isEmpty() || confirmEmail.isEmpty()) {
            App.showMessage("Faltan campos por llenar", this);
        } else if (countLetterMay <= 0 && countLetterMin <= 0 && countNumber <= 0 && countSimbol <= 0) {
            App.showMessage("Almenos una letra minuscula, una mayuscula, un numero y un caracter", this);
        } else if (userName.length()>12){
            App.showMessage("El username debe ser menor o igual a 10 caracteres", this);
            username.setText("");
        } if (passwordFirst.length()< 8){
            App.showMessage("La contraseña debe ser mayor a 8 caracteres", this);
        } else if (!passwordFirst.equals(confirmPassword)) {
            App.showMessage("Las contraseñas no coinciden", this);
        } else if (!emailFirst.equals(confirmEmail)) {
            App.showMessage("El correo no coincide", this);
        } else {
            SharedPreferencesController sharedPreferences = new SharedPreferencesController(this);
            sharedPreferences.saveUser(
                    App.createUser(names,lastName,userName,passwordFirst,emailFirst)
            );
            App.showMessage("Usuario creado", context); //Aqui me indica el error...
            finish();
        }
    }

<Button
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:layout_margin="10dp"
            android:background="@color/colorPrimary"
            android:text="Register"
            android:onClick="saveData"  //Metodo
            android:textColor="@android:color/white"
            android:textStyle="bold" />

public class SharedPreferencesController {
private SharedPreferences sharedPreferences;

private final String names = "nameKey";
private final String lastName = "lastNameKey";
private final String userName = "userNameKey";
private final String email = "emailKey";
private final String userId = "userId";

public SharedPreferencesController(Context context){
    sharedPreferences = context.getSharedPreferences(context.getPackageName(), Context.MODE_PRIVATE);
}

public void saveUser(User user){
    SharedPreferences.Editor editor = sharedPreferences.edit();
    editor.putString(names, user.getName());
    editor.putString(lastName, user.getLast_name());
    editor.putString(userName, user.getUsername());
    editor.putString(email, user.getEmail());
    editor.putInt(userId, user.getUser_id());
    editor.commit();
    Log.i("SharedPtreferences","Se registro"+user.getUser_id());
}