collection is not bindable, i.e. the property which exposes it is read-only. You have found the right approach of defining whether an item is checked or not by using the ItemCheckedPath
As for the Toolbox issue, we are not sure why it happens. Can you please try opening a single instance of Visual Studio, add the controls to the Toolbox and close that instance so that all settings are stored. After that you should continue working without having any issues according to our experience.
Let me know if this helps.
the Telerik team
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