This usually happens if you had an older version of the WPF Demos app and you install the new one on top of that. Sometimes the ClickOnce installer is caching files which leads to issue in the demo. To ensure that the newly installed demo app will work properly, you can clear the previous installation before that and install the new one again. You can see how to do this in the following article: https://docs.telerik.com/devtools/wpf/getting-started/installation/installing-wpf-demos#common-issues
Basically, you can delete the contents of the "C:\Users\<your_user>\AppData\Local\Apps\2.0" folder and then install the demo again.
Regards,
Martin Ivanov
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