Hi Miłosz Cechnicki,
Thank you for reporting this bug. It is logged in our public issue tracking system, its title is:
TreeView: DragDrop when bound to XML throws a null ref exception
The Elements() call returns an enumeration (i.e. not a List) and cannot be modified. The TreeView is missing some checks for this and we are going to introduce them - then DragDrop will not be allowed with non-list enumerations.
Currently I can suggest disabling the DragDrop since it should not work anyway.
If you want to enable DragDrop I can suggest presenting your XElements in an ObservableCollection and then converting them back to an XML hierarchy when needed.
Your Telerik Points have been updated for your feedback.
the Telerik team
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