Fix crash on select2 widgets having already a value set for the 'width' property before 2.16

See: 'participants' widget on the first node for both 'ParallelDocumentReview' and 'SerialDocumentReview' workflows.

https://connect.nuxeo.com/nuxeo/site/studio/ide?project=nuxeo-routing-default#@settings
also visible on:
https://connect-test.nuxeo.com/nuxeo/site/studio/ide?project=nuxeo-routing-default#@feature:SerialDocumentReview.workflow
Can not reproduce it when adding a new widget.