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The most common cause for such behavior is because of inappropriate use of NoXAML binaries. Could you try removing all references and adding them anew, cleaning and rebuilding the project? Please make sure the new references should be the ones containing XAML and all referenced .dll files should be the same version. Please take a look at this article
about all needed Assembly References to start using RadRichTextBox for WPF.
Also, you could find more information about XAML and NoXAML binaries at this help article