conexionBD cc = new conexionBD();
    Connection cn=cc.conectar;
    String sql="";

    if(rbnHerraOfi.isSelected())
        tip=rbnHerraOfi.getText();
    else
        tip=rbnArtPape.getText();
    sql="INSERT INTO productos (Articulo,Precio_compra,Precio_venta,Stock_min,TipoArticulo,Cantidad) VALUES (?,?,?,?,?,?)";

    try {
         PreparedStatement pst = cn.prepareStatement(sql);
         pst.setString(1,txt_NombrePro.getText());
         pst.setDouble(2,Double.parseDouble(txtPrecioCompraPro.getText()));
         pst.setDouble(3,Double.parseDouble(txtPrecioVentaPro.getText()));
         pst.setInt(4,Integer.parseInt(txtStockMinimo.getText()));
         pst.setString(5,tip);
         pst.setInt(6,Integer.parseInt(txtCantidadPro.getText()));

         int n=pst.executeUpdate();
         if(n>=0){
             JOptionPane.showMessageDialog(null,"Registros Guardados con Exito");
              limpiar();
         }

    } catch (SQLException ex) {
        JOptionPane.showMessageDialog(null,""+ex);

    }
        `

public class conexionBD {
Connection conectar=null;

public Connection conectar() {

    try {
        Class.forName("org.gjt.mm.mysql.Driver");
        conectar=DriverManager.getConnection("jdbc:mysql://localhost/papeleria","root","");
        JOptionPane.showMessageDialog(null, "Conectado correctamente");
    } catch (Exception e) {
        JOptionPane.showMessageDialog(null, "Error al conectarse a la base de datos"+e);
    }
    return conectar;
}

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
at pepeleria2.Admin2.btnGuardarProActionPerformed(Admin2.java:650)

if(rbnHerraOfi.isSelected()) {
        tip=rbnHerraOfi.getText();
   } else {
        tip=rbnArtPape.getText();
        sql="INSERT INTO productos (Articulo,Precio_compra,Precio_venta,Stock_min,TipoArticulo,Cantidad) VALUES (?,?,?,?,?,?)";
   }