Why is my form blinking

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();
}
}
}
}
this my form code.