Most likely, you get nested update panels - one around the RadWindow and one inside the ContentTemplate. Thus, when the browser starts evaluating the HTML for the inner one it turns out that the inner one was already disposed by the outer one.
By update panel I mean an asp:UpdatePanel or a RadAjaxPanel or an AJAX setting from a RadAjaxManager/RadAjaxManagerProxy that encompasses the RadWindow declaration.
Thus, I would advise the following:
- examine this sticky thread to open the RadWindow via registering a script from the server: http://www.telerik.com/community/forums/aspnet-ajax/window/opening-radwindow-from-the-server.aspx. This will let you remove the update panel around the RadWindow.
- examine this thread on using AJAX in this case: http://www.telerik.com/help/aspnet-ajax/radwindow-ajaxifying.html. Once again, only an updatePanel in the ContentTemplate should be needed.
- Look for other nested AJAX settings/update panels and remove this nesting because it can cause other issues too. This article can help you get started - http://www.telerik.com/help/aspnet-ajax/ajax-controls-in-ajaxpanel-and-ajaxsettings.html - it treats RadAjaxPanels and RadAjaxManager, but the approach for asp:UpdatePanels and RadAjaxManager settings is the same.
You can take this further and consolidate the needed ajax setup on the page to avoid nesting in other places.
I hope this helps.
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