test.cs

public IActionResult Editor(string id = null)
{
    EditorViewModel model = new EditorViewModel()
    {
        InvoiceData = InvoiceData.GetDummyInvoiceData(),
        DocumentId = id
    };

    if (id != null)
    {
        // url encoded
        id = id.Replace("%2F", "/");

        model.TemplateData = Storage.GetTemplate(id);
    }

	return View(model);
}