Closed Unassigned: Cannot create an instance of "ViewModelLocator". [7651]

I have a WPF4 project with MVVMLight libraries ver 4.3.31.28629 and the designer throw the error
Cannot create an instance of "ViewModelLocator".

I also tried with a blank project with only the MVVMlight libraries installed via nuget and still got the error.

I need to fix this because I highly need to use design time models