To reproduce: 
1. Drag and drop RadPdfViewer and RadPdfViewerNavigator
2. Load attached file from ticket
3. Click PrintPreview button from navigator and you will see that NullRefference exception is thrown. 

Workaround: 
You can set the ShowPrintPreview property to false of RadPrintViewNavigator
this.radPdfViewerNavigator1.ShowPrintPreview = false;