Steps to reproduce:
in a space go to Space Tools → Apps → Viewport,
activate a viewport with the default theme → uncheck all options for content type,
save the configuration → access the viewport URL for this space.
the error from the preview is shown, or better
it is not possible to save a viewport configuration unless at least one content type is selected.
stacktrace appears when trying to access Viewport URL
go to Space Tools → Apps → Viewport → enable at least one of the content types (pages, blogs)