Under Locations ⇾ Terminals, click "Add new terminal"
Make sure that the hardware address field is empty*.
Click "Add printer". The details view closes, and the terminal is gone.
*NOTE: There is a space as hardware address by default, this needs to be removed.
It's the same problem when adding new locations
The expected behavior is probably that you should be able to add printers before you decide on a name or hardware address. Since you haven't pressed "Save" yet, I would expect to be able to edit the Terminal or Location as I please.
Should work now. Note that the overwrite phenomenon documented in bug 8084 is still a problem.
I still have the problem on locations page. When adding a new location, giving it an empty name, and clicking "Add printer" the popup is closed. It is not as bad as terminals page was before, since the name is not empty by default for locations.
Created a new bug for locations page, bug 8123.
The terminals page works just fine now. Tested the original problem, and it is now possible to add a printer when hardware address is empty. Note also that the "hidden" space mentioned in comment #0 has been removed as a part of this bug and bug 8075.