TypeNameParserException is printed in the Output pane of Visual Studio, when RadNavigationView is initialized. The exception message is:
MS.Internal.Xaml.Parser.GenericTypeNameParser.TypeNameParserException: 'Prefix 'helpers' does not map to a namespace.'
This happens because of a known issue in the Visual Studio designer, which is discussed here.
This issue can be safely ignored. It won't cause issues in the control at runtime.