When I run admin login form , then nullpointerexception error appear.please solve this my issue.thanks

package Admin_Login;
import AdminPanel.AdminSection;
import database.db;
import java.sql.*;
import javax.swing.JOptionPane;
public class AdminLogin extends javax.swing.JFrame
{
     Connection conn;
     PreparedStatement pst;
     ResultSet rs;
     AdminSection as;
     public AdminLogin()
     {
         initComponents();
     }
     private void LoginActionPerformed(java.awt.event.ActionEvent evt) {
     String sql="select id,username,password from Users where(username=? and password=?)";
     try
      {
           int count =0;
           pst = conn.prepareStatement(sql);
           pst.setString(1, username.getText());
           pst.setString(2, password.getText());
           rs =pst.executeQuery();
           while(rs.next())
            {
               count =count+1;
            }
           if(count==1)
                {
                    JOptionPane.showMessageDialog(null, "Success Login");
                    as= new AdminSection();
                    as.setVisible(true);
                    this.dispose();
                }
           else
                {
                   JOptionPane.showMessageDialog(null, "Username and
                    Password you entered are not valid");
                }
       }
       catch(Exception e)
       {

       }
        finally
       {
           try
           {

               pst.close();
           }
           catch(Exception e)
           {
               JOptionPane.showMessageDialog(null, e);
           }
       }
    }

        /* Create and display the form */
        java.awt.EventQueue.invokeLater(new Runnable() {
            public void run()
            {
                new AdminLogin().setVisible(true);
            }
        });
    }

    // Variables declaration - do not modify
    private javax.swing.JButton Login;
    private javax.swing.JLabel jLabel1;
    private javax.swing.JLabel jLabel2;
    private javax.swing.JLabel jLabel3;
    private javax.swing.JPanel jPanel1;
    private javax.swing.JTextField password;
    private javax.swing.JTextField username;
    // End of variables declaration
    }

    package database;
    import Classes.add_Librarian;
    import java.sql.*;
    import javax.swing.*;
    public class db 
    {
        private Connection conn;
        private Statement st;
        private ResultSet rs;
        public  static Connection establishConnection()
        {
            try
            {
               Class.forName("com.microsoft.sqlserver.jdbc.SQLServerDriver");
             Connection conn=DriverManager.getConnection("jdbc:sqlserver://localhost\\sqlexpress:1433; database=Employee Payroll; user=pak; password=123");
               if(conn==null)
               {
                   JOptionPane.showMessageDialog(null, "Not Connected");
               }
               else
               {
                   JOptionPane.showMessageDialog(null, "Connected");
               }
               
               return conn;
            }catch(Exception ex)
            {
               JOptionPane.showMessageDialog(null,"Connection not established");
               return null;
            }
        }
        public int addlibrarian (add_Librarian lb)
        {
            int n=0;
            try
            {
                n=st.executeUpdate("insert into Add_Librarian values ('"+lb.getName()+"','"+lb.getFatherName()+"',"
                        +lb.getID_CardNo()+"','"+lb.getQualification()+"','"+lb.getContactNo()+"','"+lb.getAddress()+"')");
            }
            catch(Exception e)
            {
                JOptionPane.showMessageDialog(null,e);
            }
            return n;
        }
        public void closeConnection()
         {
             try
             {
                 conn.close();
             }
             catch(Exception ex)
             {
                        JOptionPane.showMessageDialog(null,ex);
        
             }
         }
    }