[BUG] "Duplicate Top Stack Value" custom command option does not work

[clechasseur]

Describe the bug
Including a "Duplicate Top Stack Value" element in a custom command (through Expert Mode) makes the command invalid (and unusable).

To Reproduce

1. Create a new custom command
2. Switch to Expert Mode
3. Add a "Duplicate Top Stack Value" element
4. In the Preview pane at the bottom right, see << Invalid custom command >>
5. Give the command a name, then click OK to save it
6. Try opening the contextual menu on a file
7. Go to the Path Copy Copy submenu
8. See that the new custom command is grayed out

Expected behavior
The command should work (since "Duplicate Top Stack Value" does nothing if the stack is empty).