ComboBox in java doesn't work bluej

I have a problem with my ComboBox, I search everywhere and my code is fine. But when I run the program it shows me java.lang.NullPointerException error

Here's my code

    package InventarioGUI;

    import java.awt.*;
    import java.awt.event.*;
    import javax.swing.*;
    import java.sql.SQLException;
    import java.sql.PreparedStatement;
    import java.sql.*;



public class VentanaInventario extends JFrame implements ActionListener
{

    private JComboBox cmbProducto, cmbProveedor;
    ConexionInventario Con = new ConexionInventario();
    private PreparedStatement PST = null;

    DefaultTableModel md;
    JTable tabla;
    Object data [][] = {};
    String cabeza[] = {"Cantidad", "Fecha Entrada", "Precio"};
    JScrollPane scroll;    

   public VentanaInventario()
   {
       super ("Inventario");   

       ComboProducto();
       setLayout(null);

       cmbProducto = new JComboBox();
       cmbProducto.setMaximumRowCount(5);
       cmbProducto.setBounds (120, 10, 150, 20); 
       add(cmbProducto);
    }

    private void ComboProducto(){
        try
        {
           String Sql = "SELECT Nombre_Producto FROM Producto";
           Con.ExeSql(Sql);

            while(Con.RS.next()){
                String pat = Con.RS.getString("Nombre_Producto");
                cmbProveedor.addItem(pat);

            }
       }

        catch(Exception ex){
            JOptionPane.showMessageDialog(null, ex);
        }
    }

   public static void main (String args[])
   {
       try
       {
           VentanaInventario frmVentanaInventario = new VentanaInventario();
           frmVentanaInventario.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
           frmVentanaInventario.setSize (300, 250);
           frmVentanaInventario.setVisible(true);
        }

        catch (Exception ex)
        {
         JOptionPane.showMessageDialog(null, "Error presentado al realizar operación", " VentanaInventario", JOptionPane.ERROR_MESSAGE);
        }
    }
}

      

        
        
        
      

    

All the mix and everything is in a different class and everything is fine. Pleeease, help, I have days with this and I don't know what else I can do!