Required directory field validation fails in Server side on document creation

Steps to reproduce:

• Create a vocabulary,e.g. Skill:

  Id	Label
  Listening	skillListening
  Reading	skillReading
  Speaking	skillSpeaking

  *Create a custom schema, e.g. skill

  Field	Type	Required
  skillField	Directory (Skill)	yes

• Associate the schema with a Document type,e.g. Activity
• Generate default viewes.
• This code is scaffolded

      <nuxeo-directory-suggestion value="[[document.properties.skill:skillField]]" label="Skill" directory-name="Skill" role="widget" min-chars="0"></nuxeo-directory-suggestion>
  

• Deploy your changes
• Try to generate a new Activity (setting values for the fields)
• The creation fails due to a validation error in the required directory field (See attached images)

elements.html:121712 
{
  entity-type: "validation_report", has_error: true, number: 1, violations: Array(1), status: 400}
  entity-type: "validation_report"
  has_error: true
  number: 1
  status: 400
  violations: Array(1)
    0: 
      constraint: {entity-type: "validation_constraint", name: "NotNullConstraint", parameters: {…}}
      invalid_value:null
      message: "Value is required."
      path: Array(1)
        0:
          field_name: "skill:skillField"
          is_list_item: false
          __proto__: Object
          length: 1
          __proto__: Array(0)
          __proto__: Object
          length: 1
          __proto__: Array(0)
          __proto__: Object

NOTE: Workaround to get the app running while this issue is solved:

*Uncheck the "required" checkbox in the schema definition

• Add the required attribute in the nuxeo-directory-suggestion web element
• Deploy your changes
• Try to create a new Activity
• It works