java null pointer exception error...?

here is a simple program. that takes five results from the user and calculates the average. After that it rates each result out of 100. But i get pointer exception error at this two lines

if (result[i] >=90 && result[i] <= 100)

myInstance.determineGrade();

import java.util.*;

import javax.swing.JOptionPane;

import java.util.Scanner;

import java.io.*;

public class Main {

public int sum=0,i;

public double aVERAGE;

public String grade[];

public int[] result;

/** Creates a new instance of Main */

public Main() {

}

static Scanner sc = new Scanner(System.in);

/*static Scanner sc = new Scanner(System.in);*

* @param args the command line arguments

*/

public static void main(String[] args) throws IOException {

Main myInstance = new Main();

myInstance.calcAverage();

myInstance.determineGrade();

}

public void calcAverage(){

System.out.print("Please enter five test results\n");

Scanner keyboard = new Scanner(System.in);

int[] result = new int[5];

for(i=0;i<5;i++){

System.out.print("Please enter the " + (i+1) + " result \n");

result[i] = keyboard.nextInt();

sum += result[i];

}

aVERAGE = (double) sum/5;

System.out.print(aVERAGE);

}

public void determineGrade(){

String[] grade = new String[5];

for (i=0;i<5;i++){

if (result[i] >=90 && result[i] <= 100)

grade[i] = "A";

else if (result[i] >=80 && result[i] <= 89)

grade[i] = "B";

else if (result[i]>= 70 && result[i] <= 79)

grade[i] = "C";

else if (result[i] >= 60 && result[i]<= 69)

grade[i] = "D";

else grade[i] = "F";

System.out.print(" " + grade[i] );

}