This error comes up when a visual item, already part of the visual tree (i.e. is generated and has a parent, may not be visible) is added to the visual tree again.
This may be the case with the IstTemplates
1. Can you confirm that it is not a visual item
2. If it is a visual item, it is not part of the visual tree?
Hopefully this will work for you.
the Telerik team