Hi Roman Wack,
Thank you for reproting this issue. I logged it as a bug in our PITS
under the name Expander: Editing the default states in the ControTemplate of the Expander throws an exception
. You will be able to track its progress from tomorrow the latest.
In the meantime you can edit the states manually in XAML.
I also updated your Telerik points accordingly.
the Telerik team
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