My application wont close when the code tells it to close C# .NET

Cannot process request because the process has exited.

private void timer1_Tick(object sender, EventArgs e)
{
    var firstProcess = Process.Start(textBoxFirst.Text);
    int timeOne = Convert.ToInt32(MinTextBoxFirst.Text);
    Thread.Sleep(timeOne);
   // firstProcess.Kill();
   // if (!firstProcess.HasExited) firstProcess.Kill();
   // firstProcess.Kill();
   // MessageBox.Show("The Process Has Been Killed!");
    firstProcess.Close();



}

I changed up the code, removed the timer and used this code

        private void startlbl_Click(object sender, EventArgs e)
        {
            Process.Start(textBoxFirst.Text);
            Process.Start(textBoxFirst.Text);
            Process.Start(textBoxFirst.Text);
            Process.Start(textBoxFirst.Text);
            Process.Start(textBoxFirst.Text);

            var firstProcess = Process.Start(textBoxFirst.Text);
            int timeOne = Convert.ToInt32(MinTextBoxFirst.Text);
            Thread.Sleep(timeOne);
            firstProcess.Kill();
        }

and it gives me this error

An unhandled exception of type 'System.InvalidOperationException' occurred in System.dll

Additional information: Cannot process request because the process has exited.

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Runtime.InteropServices;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Forms;
using System.Diagnostics;
using System.Threading;

namespace Software_Restarter
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }

        

        public const int WM_NCLBUTTONDOWN = 0xA1;
        public const int HT_CAPTION = 0x2;

        [DllImportAttribute("user32.dll")]
        public static extern int SendMessage(IntPtr hWnd, int Msg, int wParam, int lParam);
        [DllImportAttribute("user32.dll")]
        public static extern bool ReleaseCapture();

        private void label1_Click(object sender, EventArgs e)
        {
            this.Close();
        }

        private void panel1_MouseDown(object sender, MouseEventArgs e)
        {
            if (e.Button == MouseButtons.Left)
            {
                ReleaseCapture();
                SendMessage(Handle, WM_NCLBUTTONDOWN, HT_CAPTION, 0);
            }
        }

        private void BrowseFirst_Click(object sender, EventArgs e)
        {
            OpenFileDialog ofd = new OpenFileDialog();
            if(ofd.ShowDialog() == System.Windows.Forms.DialogResult.OK)
            {
                string FilePath = ofd.FileName;
                textBoxFirst.Text = FilePath;
            }
        }

        private void BrowseSecond_Click(object sender, EventArgs e)
        {
            OpenFileDialog ofd1 = new OpenFileDialog();
            if (ofd1.ShowDialog() == System.Windows.Forms.DialogResult.OK)
            {
                string FilePath = ofd1.FileName;
                textBoxSecond.Text = FilePath;
            }
        }

        private void BrowseThird_Click(object sender, EventArgs e)
        {
            OpenFileDialog ofd2 = new OpenFileDialog();
            if (ofd2.ShowDialog() == System.Windows.Forms.DialogResult.OK)
            {
                string FilePath = ofd2.FileName;
                textBoxThird.Text = FilePath;
            }
        }

        private void startlbl_Click(object sender, EventArgs e)
        {
            Process.Start(textBoxFirst.Text);
            Process.Start(textBoxFirst.Text);
            Process.Start(textBoxFirst.Text);
            Process.Start(textBoxFirst.Text);
            Process.Start(textBoxFirst.Text);

            var firstProcess = Process.Start(textBoxFirst.Text);
            int timeOne = Convert.ToInt32(MinTextBoxFirst.Text);
            Thread.Sleep(timeOne);
             firstProcess.Kill();
            // if (!firstProcess.HasExited) firstProcess.Kill();
            // firstProcess.Kill();
            // MessageBox.Show("The Process Has Been Killed!");
            //firstProcess.Close();
        }
}
    }
}