我是一个新的程序员。如果你能帮我解决编码中的这些错误，我将不胜感激？
我随函附上错误信息和一些与错误相关的编码部分。
这些是发生的错误。

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
    at beacco.operation_det.OperationNo(operation_det.java:52)
    at beacco.operation_det.<init>(operation_det.java:34)

第52行-->public void operationno（）第34行-->operationno（）///在connect（）之后；
这是我的密码。

import java.awt.HeadlessException;
import java.io.PrintWriter;
import static java.lang.System.out;
import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.PreparedStatement;
import java.sql.ResultSet;
import java.sql.SQLException;
import java.sql.Statement;
import java.util.logging.Level;
import java.util.logging.Logger;
import javax.swing.JOptionPane;

public final class operation_det extends javax.swing.JFrame {

    public operation_det() 
    {
        initComponents();
        Connect();
        OperationNo();
    }

    Connection conn;
    PreparedStatement pst;

    public void Connect()
    {
        try
        {
            Class.forName("com.mysql.jdbc.Driver");
            conn = DriverManager.getConnection("jdbc:mysql://localhost:3306/beaccodb?zeroDateTimeBehavior=convertToNull","root","");
            JOptionPane.showMessageDialog(null, "Connected");

        }
        catch (ClassNotFoundException | SQLException | HeadlessException e)
        {
            JOptionPane.showMessageDialog(null, e+" Error in Connecting to database");
        }
    }

    public void OperationNo()
    {
        try {
        Statement st = conn.createStatement();
            ResultSet rs = st.executeQuery("select MAX(operation_mo) from operation");
            rs.next();

            rs.getString("MAX(operation_no");

            if (rs.getString("MAX(operation_no")== null)
            {
                operation_no.setText("00000");
            }
            else
            {
                long id = Long.parseLong(rs.getString("MAX(operation_no").substring(2,rs.getString("MAX(operation_no").length()));
                id++;
                operation_no.setText(String.format("%07d",id));
            }
        } 
        catch (SQLException ex) 
        {
            Logger.getLogger(operation_det.class.getName()).log(Level.SEVERE, null, ex);
        }

    }```