NZEC error

general
1

this is the problem link

import java.io.;
import java.util.;

/* Name of the class has to be “Main” only if the class is public.
By : SSD
*/

class Codechef {
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());
    }

    long nextLong() {
        return Long.parseLong(next());
    }

    double nextDouble() {
        return Double.parseDouble(next());
    }

    String nextLine() {
        String str = "";
        try {
            str = br.readLine();
        } catch (IOException e) {
            e.printStackTrace();
        }
        return str;
    }
}
private static int precedence(char ch)
{
    if (ch=='+'|| ch=='-')
    {
        return 1;
    }
    else if (ch=='*' || ch =='/')
    {
        return 2;
    }
    else if (ch=='^')
    {
        return 3;
    }
    else
    {
        return -1;
    }
}
public static void main(String[] args) throws Exception {
    FastReader sc = new FastReader();
    StringBuilder res = new StringBuilder();
    int t = sc.nextInt();
    while (t-- > 0) {
        
        int n=sc.nextInt();
        String infix = sc.next();
        StringBuilder ans = new StringBuilder();
        Stack <Character> st = new Stack<>();
        for (char ch : infix.toCharArray())
        {
            if (Character.isLetter(ch))
            {
                ans.append(ch);
            }
            else if (ch=='(')
            {
                st.push(ch);
            }
            else if (ch==')')
            {
                while(st.peek()!='(')
                {
                    ans.append(st.pop());
                }
                st.pop();
            }
            else
            {
                int p=precedence(ch);
                while (!st.isEmpty() && p<=precedence(st.peek()))
                {
                    ans.append(st.pop());
                }
                st.push(ch);
            }

        }
        while (!st.isEmpty())
        {
            ans.append(st.pop());
        }
        res.append(ans +"\n");
        
      

    }
    System.out.println(res);
}

}

here is my code in java, it is giving NZEC error with description

Exception in thread “main” java.lang.NullPointerException
at java.util.StringTokenizer.(StringTokenizer.java:199)
at java.util.StringTokenizer.(StringTokenizer.java:236)
at Codechef$FastReader.next(Main.java:26)
at Codechef$FastReader.nextInt(Main.java:35)
at Codechef.main(Main.java:78)

kindly help me to solve this.