In order to preserve its SelectedValue property, RadMenu saves it into the ViewState. Unfortunately, the ViewState is not yet available in the Page_Init event and this is why you are not able to access the aforementioned property. The ViewState loading occurs after
the Page_Init event and before
the Page_Load event. Here is the complete lifecycle:
I suggest you to hook on event that is after the LoadViewState if you need to use any of the properties of RadMenu.
I hope this makes sense.
the Telerik team