Hi N Aravind,
As I mentioned in my previous answer the used RadSplitter's configuration is not supported, and if I understand your scenario correctly, you are trying to use RadSplitter as a popup container, which is incorrect usage of the control.
The error that occurs is a side effect of the incorrect configuration of the control. After the postback caused by the button, RadSplitter is trying to restore the Docked state of the sliding pane, but is unable to do so.
If this is the case, you should reconsider the layout and the usage of RadSplitter. If not, could you please describe in more details the exact scenario?
the Telerik team
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