Fix selected vocabulary display in widget configuration when it's not in the list

Steps to reproduce:

• in a widget for a text field, chose the "vocabulary" widget type
• in custom props, type "directoryName" as key and a vocabulary id as value (must not be in the list of vocabulary that can be chosen in the popup)
• save

=> vocabulary id is taken into account in the widget, but not displayed next to the "browse" button when reopening the editor.