code: function() {
  		if (!this.isRequired) return false;
  
  		var invalid = !this.items.length;
  
  		this.isInvalid = invalid;
  		this.$control_input.prop('required', invalid);
  		this.$input.prop('required', !invalid);
  	}
docs: Update the `required` attribute of both input and control input.
  
  The `required` property needs to be activated on the control input
  for the error to be displayed at the right place. `required` also
  needs to be temporarily deactivated on the input since the input is
  hidden and can't show errors.