Resolved: Old form still display winforms c#

private void btnImport_Click(object sender, EventArgs e)
        {
            showFormSelectLanguage();
            if (CheckSheetFile() == true) {
                using (WaitingForm frm = new WaitingForm(handleExcel))
                {
                    frm.ShowDialog(this);
                }
                var dialogMessage = new DialogMessage();
                dialogMessage.ShowDialog(this);
            } else
            {
                ShowDialogNotFoundSheet();
            }
        }

private void showFormSelectLanguage()
        {
            var formSelectLanguage = new FormSelectLanguage();
            formSelectLanguage.ShowDialog(this);
        }

private void ShowDialogNotFoundSheet()
        {
            var dialogNotFoundSheet = new DialogNotFoundSheet();
            dialogNotFoundSheet.setTextContent("Not found sheet");
            dialogNotFoundSheet.ShowDialog(this);
        }

private void btnConfirmLanguage_Click(object sender, EventArgs e)
        {
            //close dialog
            this.Close();
        }

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

private bool CheckSheetFile()
        {
            var isCorrectFile = false;
            try
            {
                xlWorkBook = xlApp.Workbooks.Open(txtFilePath.Text, System.IO.FileMode.Open, System.IO.FileAccess.Read);
                var xlWorkBook1 = xlWorkBook.Sheets["SheetName"];
                isCorrectFile = true;
            }
            catch (Exception e)
            {
                return false;
            }
            return isCorrectFile;
        }

//btnImport.Click += btnImport_Click;
btnImport.Enabled = true;

//btnImport.Click -= btnImport_Click;
btnImport.Enabled = false;

btnImport.Click += btnImport_Click;
btnImport.Click += btnImport_Click;