This code is supposed to perform postfix calculations. Everything compiles fine. However, every time I run the code, I run into this error:

java.lang.NullPointerException
at postfixCalculator.calculate(postfixCalculator.java:77)
at postfixCalculator.main(postfixCalculator.java:90)

Any kind of help would be great.

Code:
import java.io.*;
import java.util.Scanner; 
import java.util.Stack;

public class postfixCalculator{

    Stack<String> calculator;
    
    public postfixCalculator(){
     
        calculator = null;
        
    }

    public String add(){
        Integer a = Integer.valueOf(calculator.pop());
        Integer b = Integer.valueOf(calculator.pop());
        Integer c = a+b;
        return c.toString();
    }
    
    public String subtract(){
        Integer a = Integer.valueOf(calculator.pop());
        Integer b = Integer.valueOf(calculator.pop());
        Integer c = b-a;
        return c.toString();
    }
    
    public String multiply(){
        Integer a = Integer.valueOf(calculator.pop());
        Integer b = Integer.valueOf(calculator.pop());
        Integer c = a*b;
        return c.toString();
    }
    
    public String divide(){
        Integer a = Integer.valueOf(calculator.pop());
        Integer b = Integer.valueOf(calculator.pop());
        Integer c = b/a;
        return c.toString();
    }

    public String calculate(String exp){
        String result;
        String[] parse = exp.split(" "); // split the string, we need separate characters in exp

        for (int i=0; i<parse.length; i++){ 
            if (parse[i].equals("+")) 
                calculator.push(add()); 
            else if (parse[i].equals("-"))
                calculator.push(subtract());
            else if (parse[i].equals("*")) 
                calculator.push(multiply());
            else if (parse[i].equals("/")) 
                calculator.push(divide());
            else
                calculator.push(parse[i]);         
        }
        result = calculator.pop();
        if (!calculator.empty()){
            System.out.println("Error");
            System.exit(0);
        }
        return result;
    }
    public static void main(String args[]) { 
        postfixCalculator pc = new postfixCalculator();
        Scanner sc = new Scanner(System.in);
        String expression = sc.nextLine();
        System.out.println("Expression evaluates to " + pc.calculate(expression));
    }
}