v.6.20 crash when export to PDF file. (#249)

Hi:

I wanted to export my use case and flow diagram to pdf, but error came up as below:

System.ArgumentException: Invalid PageSize.

參數名稱: value

於 PdfSharp.PageSizeConverter.ToSize(PageSize value)

於 PdfSharp.Pdf.PdfPage.set_Size(PageSize value)

於 SoftwareIdeasModeler.Core.Exporting.Images.PdfDiagramExporter.ExportToFile(String fileName)

於 SoftwareIdeasModeler.Dialogs.Exporting.BatchExporter.Export(List`1 diagrams)

於 SoftwareIdeasModeler.Dialogs.Exporting.BatchExportForm.okButton_Click(Object sender, EventArgs e)

於 System.Windows.Forms.Control.OnClick(EventArgs e)

於 System.Windows.Forms.Button.OnClick(EventArgs e)

於 System.Windows.Forms.Button.OnMouseUp(MouseEventArgs mevent)

於 System.Windows.Forms.Control.WmMouseUp(Message& m, MouseButtons button, Int32 clicks)

於 System.Windows.Forms.Control.WndProc(Message& m)

於 System.Windows.Forms.ButtonBase.WndProc(Message& m)

於 System.Windows.Forms.Button.WndProc(Message& m)

於 System.Windows.Forms.Control.ControlNativeWindow.OnMessage(Message& m)

於 System.Windows.Forms.Control.ControlNativeWindow.WndProc(Message& m)

於 System.Windows.Forms.NativeWindow.Callback(IntPtr hWnd, Int32 msg, IntPtr wparam, IntPtr lparam)