Hi Richard Ellis,
This problem is most probably caused by MEF which is used internally by RadRichTextBox to load some of its extensible parts, like UI, format-providers etc. By default, MEF walks up the entire reference tree and throws exception if some assemblies are missing. We changed this behavior so MEF will stop throwing this kind of exceptions starting from Q1 Beta 2011, expected till the end of the week.
As an immediate workaround, you should add all assemblies that Microsoft.Practices.Prism.Interactivity references - System, System.Core, System.Windows, System.Windows.Browser, System.Windows.Interactivity.
Don't hesitate to contact us if you have other questions.
the Telerik team
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