Fix edit of Directory/Document/User&Group field in schema editor

Steps to reproduce:

1. create a new schema
2. add a field
3. choose Directory or Document or User/Group in the dropdown menu
4. click on the Edit button
5. Observe the error message in the popup

   (TypeError) : a is null
   Uncaught exception: com.google.gwt.core.client.JavaScriptException: (TypeError) : a is null
       at Unknown.skh(https://connect.nuxeo.com/nuxeo/site/studio/ide?project=tmartins1010 line 9 > scriptElement)
       at Unknown.ckh(https://connect.nuxeo.com/nuxeo/site/studio/ide?project=tmartins1010 line 9 > scriptElement)
       at Unknown.gkh(https://connect.nuxeo.com/nuxeo/site/studio/ide?project=tmartins1010 line 9 > scriptElement)
       at Unknown.Zjh(https://connect.nuxeo.com/nuxeo/site/studio/ide?project=tmartins1010 line 9 > scriptElement)
       at Unknown.Elh(https://connect.nuxeo.com/nuxeo/site/studio/ide?project=tmartins1010 line 9 > scriptElement)
       at Unknown.Qlh(https://connect.nuxeo.com/nuxeo/site/studio/ide?project=tmartins1010 line 9 > scriptElement)
       at Unknown.Gib(https://connect.nuxeo.com/nuxeo/site/studio/ide?project=tmartins1010 line 9 > scriptElement)
       at Unknown.Hib(https://connect.nuxeo.com/nuxeo/site/studio/ide?project=tmartins1010 line 9 > scriptElement)
       at Unknown.Bhb(https://connect.nuxeo.com/nuxeo/site/studio/ide?project=tmartins1010 line 9 > scriptElement)
       at Unknown.$qb(https://connect.nuxeo.com/nuxeo/site/studio/ide?project=tmartins1010 line 9 > scriptElement)
       at Unknown.xrb(https://connect.nuxeo.com/nuxeo/site/studio/ide?project=tmartins1010 line 9 > scriptElement)
       at Unknown.Drb(https://connect.nuxeo.com/nuxeo/site/studio/ide?project=tmartins1010 line 9 > scriptElement)
       at Unknown.nrb(https://connect.nuxeo.com/nuxeo/site/studio/ide?project=tmartins1010 line 9 > scriptElement)
       at Unknown.Tz(https://connect.nuxeo.com/nuxeo/site/studio/ide?project=tmartins1010 line 9 > scriptElement)
       at Unknown.Mhb(https://connect.nuxeo.com/nuxeo/site/studio/ide?project=tmartins1010 line 9 > scriptElement)
       at Unknown.Zz(https://connect.nuxeo.com/nuxeo/site/studio/ide?project=tmartins1010 line 9 > scriptElement)
       at Unknown.ond(https://connect.nuxeo.com/nuxeo/site/studio/ide?project=tmartins1010 line 9 > scriptElement)
       at Unknown.nnd(https://connect.nuxeo.com/nuxeo/site/studio/ide?project=tmartins1010 line 9 > scriptElement)
       at Unknown.tsd(https://connect.nuxeo.com/nuxeo/site/studio/ide?project=tmartins1010 line 9 > scriptElement)
       at Unknown.sI(https://connect.nuxeo.com/nuxeo/site/studio/ide?project=tmartins1010 line 9 > scriptElement)
       at Unknown.vI(https://connect.nuxeo.com/nuxeo/site/studio/ide?project=tmartins1010 line 9 > scriptElement)
       at Unknown.uI/<(https://connect.nuxeo.com/nuxeo/site/studio/ide?project=tmartins1010 line 9 > scriptElement)
       at Unknown.anonymous(Unknown)