This error occurs mostly when there is a difference between the referenced Telerik assembly versions. To resolve it I would recommend you to double check if the projects in the solution references the dlls from the correct place. This should be a folder containing NoXaml dlls. The demos solution is set up to get its dlls from the UI for WPF installation folder/Binaries.NoXaml/WPF40/.
So, if you have installed the UI for WPF controls the solution should run properly when you run it. You can also try to delete the 'Deploy' folder from the demos solution and then rebuild the solution again.
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