Hello Zacharias Karasavvas,
I am sorry for the delayed reply.
The Telerik TreeView does not work with a toolkit HierarchicalDataTemplate, we use our own:
This is because we do not want to create a dependency on a toolkit assembly and we can support more features like ItemTemplateSelectors and ItemContainerStyleSelectors.
Do come back to us if you need more help.
the Telerik team
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