ถามเกี่ยวกับการเปิดปิด child form หน่อยครับของผมพอเปิดรอบ 2 มัน error

ublic partial class FormMain : Form
    {
        private FormStore formStore = new FormStore();
       // private int checkStore;
        public FormMain()
        {
            InitializeComponent();
        }

        private void FormMain_Load(object sender, EventArgs e)
        {
            this.Width = 1200;
            this.Height = 750;
            this.Left = (int)(Screen.PrimaryScreen.Bounds.Width / 2) - (int)(this.Width / 2);
            this.Top = 5;
        }
        
        private void toolStrip1_ItemClicked(object sender, ToolStripItemClickedEventArgs e)
        {
            string txt = e.ClickedItem.Text;
            switch (txt)
            {
                case "รายการสินค้า":
               {
                       ShowChildForm(formStore);   
               }
                    break;
            }
        }
        private void ShowChildForm(Form f)
        {
            if(f.Visible == true)
            {
                f.BringToFront();
            }
            else
            {
                f.MdiParent = this;
                f.Show();
            }
        }

        private void FormMain_MdiChildActivate(object sender, EventArgs e)
        {
            if (this.ActiveMdiChild == null)
            {
                return;
            }
            else
            {
                toolStripStatusLabel1.Text = this.ActiveMdiChild.Text;
            }
        }
    }