Why does the Visualizer say that every atom is hidden?

Below is a model of persons and their fathers. I want the Alloy Analyzer to show an instance of the model. In the Alloy tool I select "Run Show" (in the Execute menu). Then I select the "Show" button. Here's what the Visualizer displays:

I don't know what that means. I clicked on Theme and I see nothing to help me "unhide" the atoms. Help! Why is the Visualizer showing this?

sig Person {father: Person}

fact {
    no p: Person | p in p.^father
}

pred Show {}

run Show

It means that either there are no atoms in the solution, or you have some but they're hidden by the theme. In this case, it seems the instance is one in which there are no atoms. We should correct that message...