java: Problem on second if condition , NullPointerException [duplicate]

i'm trying to make a printDetails(); method but i keep getting an error "NullPointerException"

the method is on class Customer on the second if condition i tried replacing the condition statements, and tried declaring the calculateTotalPrice(); as a variable, and tried else if but all didn't work

CODE

public class Main {

public static void main(String[] args) {
    Studio studio1 = new Studio(1,2799,0.1);
    Customer c1 = new Customer("02","Naif", studio1);

    Suite suite1 = new Suite(1, 3200, "Free breakfast");
    Customer c2 = new Customer("02", "Faisal", suite1);

    c1.printDetails();
    c2.printDetails();

the customer class

    public void printDetails() {
    if (studio.isEmpty()){
        System.out.println("Customer '" + name + "' ID: " + id + ", Reserved a studio with a total of '" + studio.calculateTotalPrice()+ "'");
    }

    if (suite.isEmpty()){
        System.out.println("Customer '" + name + "' ID: " + id + ", Reserved a suite with a VIP service of '" + suite.VIPServices+ "'");
    }
}

the isEmpty() method:

public boolean isEmpty(){
        return true;
    }

EDIT:

i solved it using a flag

in customer class i added public int flag

and on first const flag =1

second const

flag =2