NullPointerException

> NullPointerException:
  at Expand.fib(Expand.java:53) = sorter[counter] = (sorter[counter] + filename.charAt(i));
  at Expand.main(Expand.java:16) = sorter1 = fib("header.h");
  at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
  at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
  at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
  at java.lang.reflect.Method.invoke(Unknown Source)

import java.io.*;
import java.util.*;
import java.lang.*;

public class Expand {
  private static String word;
  private static Scanner s;
  private static String holder;
  private static String a;
  private static final String pattern = "#include";
  private static String filename = "";
  private static String [] sorter;
  private static String [] sorter1;
 
  public static void main(String[] args) throws IOException {
    sorter1 = fib("header.h");
   /*
    * Recursion method here
    */
   

    System.out.println(sorter1);
   }
 
  public static String getToken() {
    word = s.next();
    word = word.replaceAll("[\\*/\"]", "");
    return word;
  }
 
  public static String [] fib(String file) throws IOException {
    s = new Scanner(new BufferedReader(new FileReader(args[0]+file)));
    while (s.hasNext()) {
      holder = holder + (a=getToken());
     
      if(a.matches(pattern)) {
        if(filename == null) {
          filename = getToken();
        }
       
        if(filename != null) {
          filename = filename + " " + getToken();
          //System.out.println(filename);
        }
      }
    }
    int counter = 0;
    for(int i = 0; i < filename.length(); i++) {
      if(filename.charAt(i) == ' '){
        counter++;
      }
      if(filename.charAt(i) != ' ') {
        sorter[counter] = (sorter[counter] + filename.charAt(i));
      }
    }
    s.close();
    return sorter;
  }
}