To reproduce: 
1. Add a new form 
2. Drag and drop RadApplicationMenu on the form
3. Add RadMenuSeparatorItem at design time 
4. Save the form
5. Press Ctrl + Z to undo changes 
6. Press Ctrl + Y to redo changes 
7. An exception is thrown with message: Object of type 'System.Boolean' cannot be converted to type 'Telerik.WinControls.ElementVisibility'.