The type 'Window' does not support direct content

Question

I have a WPF solution built with VS 2015 composed of several projects. Suddenly I started receiving a warning in design mode stating the following:

The t

Answer 1

on behalf of @mark Richman I edited the Itemtemplate to automatically Reference "System.Xaml". Just in case some is interested:

can be found in: "[VS InstallDir]\Common7\IDE\ItemTemplates\VisualBasic\WPF\[InputLocale]\WPFWindow"

BR, Daniel