We are sorry to hear that you have this issue. Thank you for providing a source code, it has helped to reproduce the issue. It is a bug inside the GroupDocs.Viewer, an exception occurs when there is one and only one item in the array (or any other sequence like list, the specific type does not matter). If there is more then one item, the exception doesn’t occur and GroupDocs.Viewer shows several documents.
Our developers are working on this bug. We will notify you in this forum thread when it will be fixed.
Thanks and sorry for the inconvenience.