The bug is that currently we can’t deselect the default selected option from the select field choices popup. The checkbox don’t handle the second click to uncheck it.
To be able to solve, I created another option, checked it then removed the option.
The reason why it’s a bug is that we can use the placeholder, instead of a option with no value, as first option. Having both configured, placeholder and a checked option, renders two ‘selected’ options at the frontend.