Dynamique bouton// problème d'instance

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Forms;
using System.Globalization;
using System.Reflection;
using DevExpress.XtraPrinting;
using DevExpress.XtraTabbedMdi;
using DevExpress.XtraEditors;
using System.Dynamic;
using System.IO;
using System.Data.OleDb;
using System.Linq.Expressions;
using System.Runtime.InteropServices;
using System.Data.SqlClient;


namespace Parc_Auto_V2019
{
    public partial class MenuPrincipal : DevExpress.XtraBars.Ribbon.RibbonForm
    {
        SqlConnection conn = new SqlConnection();
        SqlCommand cmd = new SqlCommand();
        public MenuPrincipal()
        {
            InitializeComponent();
        }
        private void Viewform(Form _form)
        {
            _form.MdiParent = this;
            _form.Show();
        }

        private void barButtonItem4_ItemClick(object sender, DevExpress.XtraBars.ItemClickEventArgs e)
        {
            Affectation Af = new Affectation();
            Af.Show();
        }

        private void barButtonItem5_ItemClick(object sender, DevExpress.XtraBars.ItemClickEventArgs e)
        {

        }

        private void barButtonItem6_ItemClick(object sender, DevExpress.XtraBars.ItemClickEventArgs e)
        {
            Mecanicien Mc = new Mecanicien();
            Mc.Show();
        }

        private void barButtonItem15_ItemClick(object sender, DevExpress.XtraBars.ItemClickEventArgs e)
        {
            Entretiens En = new Entretiens();
            En.Show();
        }

        private void barButtonItem16_ItemClick(object sender, DevExpress.XtraBars.ItemClickEventArgs e)
        {
            Toleries Tl = new Toleries();
            Tl.Show();
        }

        private void barButtonItem17_ItemClick(object sender, DevExpress.XtraBars.ItemClickEventArgs e)
        {
            Assurance As = new Assurance();
            As.Show();
        }

        private void barButtonItem19_ItemClick(object sender, DevExpress.XtraBars.ItemClickEventArgs e)
        {
            Service Sr = new Service();
            Sr.Show();
        }

        private void barButtonItem8_ItemClick(object sender, DevExpress.XtraBars.ItemClickEventArgs e)
        {
            Vehicule Vh = new Vehicule();
            Vh.Show();
        }

        private void barButtonItem9_ItemClick(object sender, DevExpress.XtraBars.ItemClickEventArgs e)
        {
            Marque Mr = new Marque();
            Mr.Show();
        }

        private void barButtonItem10_ItemClick(object sender, DevExpress.XtraBars.ItemClickEventArgs e)
        {
            Modele Md = new Modele();
            Md.Show();
        }

        private void barButtonItem14_ItemClick(object sender, DevExpress.XtraBars.ItemClickEventArgs e)
        {
            Aff_Vehicules Av = new Aff_Vehicules();
            Av.Show();
        }

        private void barButtonItem1_ItemClick(object sender, DevExpress.XtraBars.ItemClickEventArgs e)
        {
           frmChild1 frm = new frmChild1();
            frm.MdiParent=this;
            frm.Show();
           
        }

        private void barButtonItem2_ItemClick(object sender, DevExpress.XtraBars.ItemClickEventArgs e)
        {
            frmChild2 frm = new frmChild2();
             frm.MdiParent = this;
            frm.Show();
            Viewform(frm);
        }

        private void frmChild3_ItemClick(object sender, DevExpress.XtraBars.ItemClickEventArgs e)
        {
            
        }

    }
}