Thank you for the explanation.
It seems that the viewstate for your grid is not properly persisted across postback requests but unfortunately I am not able to determine the exact reason for that without examining your code implementation in detail. Can you please post the code related to the grid instantiation in your web part in this forum post?
In the meantime please verify that you instantiate the grid structure programmatically in accordance with the concepts explained in the following documentation article:
In addition, you may be interested in gathering more general information about how to embed RadGrid in Sharepoint WebPart:
the Telerik team