Hello ghader hosseinpour,
Please allow me to add a bit more information to your issue.
In Q3 2009 we refactored our Telerik Presentation Framework which brought signification performance and memory improvements. For additional information, please refer to the following article: RadElement Lifecycle – a WinForms TPF Story
In the new TPF version we removed the EnsureChildItems() method. So, you need to recompile your application with the new Telerik assemblies removing the EnsureChildItems call from your code.
the Telerik team