Null pointer exception

package com.codegym.task.task09.task0930; import java.io.BufferedReader; import java.io.*; import java.util.*; /* Task about algorithms */ public class Solution { public static void main(String[] args) throws Exception { BufferedReader reader = new BufferedReader(new InputStreamReader(System.in)); ArrayList<String> list = new ArrayList<String>(); while (true) { String s = reader.readLine(); if (s.isEmpty()) break; list.add(s); } String[] array = list.toArray(new String[list.size()]); sort(array); for (String x : array) { System.out.println(x); } } public static void sort(String[] array) { // write your code here String tmp; for(int i=0;i<array.length-2;i++) { if(!isNumber(array[i])) continue; for(int j=i+1;j<array.length;j++) { if(!isNumber(array[j])) continue; if(isGreaterThan(array[j],array[i])) { tmp=array[j]; array[j]=array[i]; array[i]=tmp; } } } for(int i=0;i<array.length-2;i++) { if(isNumber(array[i])) continue; for(int j=i+1;j<array.length;j++) { if(isNumber(array[j])) continue; if(isGreaterThan(array[i],array[j])) { tmp=array[j]; array[j]=array[i]; array[i]=tmp; } } } } // String comparison method: 'a' is greater than 'b' public static boolean isGreaterThan(String a, String b) { if(isNumber(a)) return Integer.parseInt(a)>Integer.parseInt(b); return a.compareTo(b) > 0; } // Is the passed string a number? public static boolean isNumber(String s) { if (s.length() == 0) return false; char[] chars = s.toCharArray(); for (int i = 0; i < chars.length; i++) { char c = chars[i]; if ((i != 0 && c == '-') // The string contains a hyphen || (!Character.isDigit(c) && c != '-') // or is not a number and doesn't start with a hyphen || (i == 0 && c == '-' && chars.length == 1)) // or is a single hyphen { return false; } } return true; } }