I think this is possible. At a minimum, any code defined in the menu formula will run before the PopUpButtonObject has finished processing. I would recommend not using code in menus associated with the PopupButtonObject. This feature (the CODE option) was designed for use with the regular menu bar.
The PopupButtonObject is really not designed for performing commands, as you are doing – it is designed for changing a data value. Basically a variation on using a bunch of radio buttons. You wouldn’t use radio buttons for a command like Set default truck.
If you want to perform commands from a popup menu, I would recommend using a shape object, an image object, or a text display object with a Font Awesome icon, and triggering the popup menu with the popupclick statement, or a variation.