While Running This Code I Got Error As Exception In Thread "Main" Java.Lang.Nullpointerexception At Test.Main(Test.Java:15)..Wta Should I Do?

import java.io.*;
import java.io.FileReader;
import java.io.BufferedReader;
import java.io.PrintWriter;
class Test {
public static void main(String[] args)throws IOException
 {
    BufferedReader input;
    String line;
    File intermediate = new File("myfile1.txt");
    PrintWriter output = new PrintWriter(intermediate, "UTF-8");
    String p = "hh";
    File dir = new File(p);
    File[] files = dir.listFiles();
    for (File f : files) 
{
      if (f.isFile())
 {
        input = new BufferedReader(new FileReader(f));
        line = String.format("[%s_DINAMALAR_DATE]", f.getName());
        output.println(line);
        System.out.println(line);
        while ((line = input.readLine()) != null) 
{
          output.println(line);
          System.out.println(line);
        }
        input.close();
      }
    }
    output.close();
    input = new BufferedReader(new FileReader(intermediate));
    int count = 0;
    output = null;
    while ((line = input.readLine()) != null) 
{
      if (line.startsWith("["))
 {
        if (output != null) output.close();
        output = new PrintWriter(new File(line + ".txt"), "UTF-8");
        output.println(line);
        count++;
        continue;
      }
      output.println(line);
    }
    input.close();
  }
}