Just to be clear for anyone else coming across this the reference to the namespace was needed.
Eg the xmlns:toolkit line below:
Eg the xmlns:toolkit line below:
<Applicationxmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"xmlns:toolkit="http://schemas.xceed.com/wpf/xaml/toolkit"x:Class="MyClass.App">