RadAjaxPanel doesn't init child RadGrid control

This is the basic page set-up I have:

• MasterPage: Contains RadScriptManager and RadAjaxManager
• Page: Contains User Control (a)
  
• User Control (a): Contains RadTreeView and User Control (b)
• User Control (b): Contains RadAjaxPanel which contains RadGrid

The RadTreeView is configured in code-behind to trigger the RadAjaxPanel.

Now, when the RadAjaxPanel loads the RadGrid, it doesn't call Sys.Application.on_init(RadGrid), so the RadGrid is no more a member of Sys.Application._components and $find(RadGrid) returns null when called from the RadAjaxManager's OnResponseEnd callback.

What do I need to do for having $find(RadGrid) find the RadGrid in the OnResponseEnd callback?