code: function isFieldStillValidTarget(fieldOrPageIdToCheck, pages, cb) {
        var fieldExistsInForm = false;
        var invalidPages = _.filter(pages, function(page) {
          var currentPageId = page._id.toString();
  
          if (currentPageId === fieldOrPageIdToCheck) {
            fieldExistsInForm = true;
          }
          var invalidFieldList = _.filter(page.fields, function(field) {
            var currentFieldId = field._id.toString();
            if (currentFieldId === fieldOrPageIdToCheck) {
              //Current field exists
              fieldExistsInForm = true;
  
              //Field is admin only, therefore it is an invalid target for a rule.
              if (field.adminOnly) {
                return true;
              } else {
                return false;
              }
            } else {
              return false;
            }
          });
  
          //If the invalidFieldList is > 0, it means that one of the fields was invalid.
          return invalidFieldList.length > 0;
        });
  
        var invalidField = invalidPages.length > 0;
  
        //Invalid if either the field is invalid, or it does not exist in the form.
        if (invalidField === true || !fieldExistsInForm) {
          return cb(true);
        } else {
          return cb();
        }
      }
docs: A field target is not valid if it does not exist in the form or it is an adminOnly field.
  @param fieldOrPageIdToCheck
  @param pages
  @returns {boolean}