Csharp 旧表单仍然显示winforms c#

当单击按钮"开始"时，我实现了一个处理excel文件的表单.

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();
            }
        }

showFormSelectLanguage方法 Select 语言的显示对话框:

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

存在用于判断表excel的ShowDialogNotFoundSheet函数:

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

事件单击" Select 语言"窗体上的"确认 Select 语言"按钮:

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

关闭对话框NotFoundSheet窗体的事件单击关闭按钮:

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

CheckSheetFile方法:

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;
        }

Issue:

Update: All References btnImport_Click:

UI: