Why I am getting null pointer exception?

#1 
import java.util.*;

import java.io.*;
public class Main {
static class FastReader
{
BufferedReader br;
StringTokenizer st;

    public FastReader() 
    { 
        br = new BufferedReader(new
                 InputStreamReader(System.in)); 
    } 

    String next() 
    { 
        while (st == null || !st.hasMoreElements()) 
        { 
            try
            { 
                st = new StringTokenizer(br.readLine()); 
            } 
            catch (IOException  e) 
            { 
                e.printStackTrace(); 
            } 
        } 
        return st.nextToken(); 
    } 

    int nextInt() 
    { 
        return Integer.parseInt(next()); 
    } 

} 
public static void main (String args[]){
	FastReader s=new FastReader(); 
    int t = s.nextInt(); 
		while(t-->0){
			int n=s.nextInt();
			int []arr=new int[n];
			for(int i=0;i<arr.length;i++){
				arr[i]=s.nextInt();
			}
			int ans=fnosas(arr);
			System.out.println(ans);
		}
	}

	public static int fnosas(int []arr){
		HashMap<Integer,Integer> maps=new HashMap<Integer,Integer>();
		int[]prefix=new int[arr.length];
		prefix[0]=arr[0];
		for(int i=1;i<arr.length;i++){
			prefix[i]=prefix[i-1]+arr[i];
		}
	
		for(int i=0;i<prefix.length;i++){
			maps.put(prefix[i], maps.getOrDefault(prefix[i], 0) + 1);
		}

		int count=0;
		for(int key:maps.keySet()){
			int val=maps.get(key);
			if(val>1){
				count+=val*(val-1)/2;
			}
		}
		return count;
	}

}