Thanks for taking interest in GroupDocs.Viewer for .NET.
You are getting this error because of using legacy document viewer application (version 2.x) with next generation GroupDocs.Viewer for .NET API (version 3.x).
The next generation GroupDocs,Viewer for .NET, starting version 3.0.0, is a back-end document processing API which renders the supported files formats into HTML pages or images. These rendered HTML pages and images can be viewed using some front end application. In contrast with the legacy versions, the front end UI has been disintegrated from the back-end API. Therefore, the users are free to create their own customized front end applications.
We have also developed the open source document viewer applications using the latest version of the API. These applications are available for the users/developers to use or modify according to their requirements. I am listing down the open source document viewer applications along with their download links.
Following are some useful resources for you to get started with next generation GroupDocs.Viewer for .NET.
If you will have any other questions, please feel free to share with us.