This is not a bug but an error which occurs under specific circumstances, when the project is not configured correctly.
There are several reasons for this error to appear and they are discussed here:
http://www.telerik.com/help/radcontrols/prometheus/?Troubleshooting.html (the last paragraph)
Please verify that you handled these cases and the error you receive should be eliminated.
I also recommend to examine our help topic which Tsvetie has suggested in her post.
Let me know whether this fixes the problem. In case this does not help I would really need to reproduce the issue locally. For this purpose, open a new support ticket and send me a small, working project (including your Web.config file and the bin directory with the dlls) and a screenshot of the error.
the Telerik team