Hi Niels,

Yes, I found this recently as well with one of our clients. I just haven't got round to fixing the issue.

If you create a controller as below

[PluginController("UmbracoApi")]
    [Authorize]
    public class FormsDataTypeApiController : BackOfficeNotificationsController
    {        
        public DataTypeDisplay GetByName(string name)
        {
            var dataType = Services.DataTypeService.GetDataTypeDefinitionByName(name);
            return dataType == null ? null : Mapper.Map<IDataTypeDefinition, DataTypeDisplay>(dataType);
        }
    }

And update the App_plugins/MwUmbForms.Rte/rte.controller.js to

(function () {
    function mwRteController($scope, $http) {
        "use strict";
        $scope.htmfield = [];
        $http.get('/umbraco/backoffice/UmbracoApi/FormsDataTypeApi/GetByName/?name=Form RTE').then(function (res) {
            var d = res.data
            $scope.htmfield = [{
                alias: "htmfield",
                label: "Html",
                view: "rte",
                config: {
                    editor: d.preValues[0].value
                },
                value: $scope.setting.value
            }];
            $scope.$watch("htmfield", function () {
                if ($scope.htmfield != undefined) {
                    $scope.setting.value = $scope.htmfield[0].value;
                }
            }, true);
        });
    }
    angular.module("umbraco").controller("mw.umbFormsRteController", mwRteController);})();

I will try and get the above fixed release soon.

Matt