Why is my form blinking

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C#

Why is my form blinking.
when i press a button inside the form,the form will blink and not disappear, and then press the button the form will disappear .
this the form code.

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;

namespace Company_Accounts
{
    public partial class MyMessageBox : Form
    {

        static MyMessageBox newMessageBox;
        static string Button_id;
        public static DataGridViewRow dr;
        public Timer msgTimer;
        int disposeFormTimer; 
        public MyMessageBox()
        {
            InitializeComponent();
        }

        private void MyMessageBox_Load(object sender, EventArgs e)
        {
            ControlBox = false;
            disposeFormTimer = 30;
            newMessageBox.lblTimer.Text = disposeFormTimer.ToString();
            msgTimer = new Timer();
            msgTimer.Interval = 1000;
            msgTimer.Enabled = true;
            msgTimer.Start();
            msgTimer.Tick += new System.EventHandler(this.timer1_Tick ); 
        }
        public static string ShowBox(string txtMessage, string txtTitle)
        {
            newMessageBox = new MyMessageBox();
            newMessageBox.label3.Text = txtTitle;
            newMessageBox.label1.Text = txtMessage;
            newMessageBox.ShowDialog();
            return Button_id;
        }

        private void btnlist_Click(object sender, EventArgs e)
        {
            newMessageBox.msgTimer.Stop();
            Button_id = "3";
            this.Hide();
            this.Close();
           
        }

        private void btnrenter_Click(object sender, EventArgs e)
        {
            newMessageBox.msgTimer.Stop();
            Button_id = "1";
            this.Close();
          
        }

        private void btnadd_Click(object sender, EventArgs e)
        {
            newMessageBox.msgTimer.Stop();
            
            Button_id = "2";
            this.Close();
           
        }

        private void timer1_Tick(object sender, EventArgs e)
        {
            disposeFormTimer--;

            if (disposeFormTimer >= 0)
            {
                newMessageBox.lblTimer.Text = disposeFormTimer.ToString();
            }
            else
            {
                newMessageBox.msgTimer.Stop();
                newMessageBox.msgTimer.Dispose();
                newMessageBox.Dispose();
            }
        }
    }
}

plz,plz, Anyone Can Help Me