We are already discussing the topic in the support thread you opened on the same issue. I will summarize my suggestions for the benefit of our community also here.
The error message: An error occurred while finding the resource dictionary "/Themes/Office2016/Telerik.ReportViewer.Wpf.xaml". WinCont App.xaml 58
probably occurs as the XAML designer locks the theme files. To prevent the undesired outcome, please, go to Tools
-> XAML Desginer
-> uncheck Enable XAML Designer
. Restart the Visual Studio afterwards. Take a look at the attached screenshot for additional reference.
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