Display "Select a value" only once when using selectOneRadioDirectory

In http://showcase.nuxeo.com/nuxeo/layoutDemo/selectOneRadioDirectoryWidget/preview/edit_mode , there is no default option displayed on the right hand side by default.
Click once on Do not display the default option => no change on the right hand side
Click a second time on Do not display the default option => on the right hand side, Select a value is displayed twice

Requesting that at the end, Select a value is displayed only once.