Adding a dialog box to a new graph corrupts the screen

I have a new graph built accessing a header/detail pair of new DACs.

Now I want to add a dialog box to the graph.

I use the visual editor, open my custom screen, select Dialogs and add a pop-up panel.

Then I try to add a form to the pop-up panel and I get the dreaded “An entry with the same key already exists.” That corrupts the form.

I can customize an existing form and add a dialog box to it with no issues. I can reproduce this at will so I’m guessing that I’m doing something wrong.