Ошибка Null Pointer Exception, как исправить ?

(x != i)

null
two
null
three
null
five

package com.javarush.task.pro.task05.task0508;

import java.util.Scanner;

/*
Удаляем одинаковые строки
*/

public class Solution {
    public static String[] strings;

    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        strings = new String[6];
        for(int x = 0; x < 6; x++){
            strings[x] = sc.nextLine();
        }
        for(int x = 0; x < 6; x++){
            int check = 0;
            for (int i = 0; i < 6; i++){
                if (strings[i] != null && strings[x] != null){
                    if(strings[x].equalsIgnoreCase(strings[i]) && (x != i)){
                        strings[i] = null;
                        check++;
                    }
                }
            }
            if (check > 0){
                strings[x] = null;
            }
        }
        for (String string : strings) {
            System.out.print(string + ", ");
        }
    }
}