Unfortunately, we cannot tell you what is causing this exception without seeing the whole implementation of the module. If you send us the project, we could have a look. The exact part of the code, that you have pasted, seems fine, in case you are using Sitefinity 4.0.
However, in case the version is 4.1, Duneel's suggestion is valid. Actually, in the Q1 release, we made a big change (improvement) of the way persistence classes are mapped in the database. You can find more information here:
The actual migration shouldn't take much time if you follow the steps. Also we will release the SDK with the updated sample till the end of this week.
the Telerik team
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