Example code for property editor with custom prevalues in v8

Can anyone point me to some sample code for property editor for v8 with prevalues? I am migrating similarity package v7 to v8. As it stands the save of prevalues is not working i get the following error:

angular.js?cdv=1:15536 Possibly unhandled rejection: {"errorMsg":"Failed to retrieve data for data type id undefined","data":{"Message":"The request is invalid.","MessageDetail":"The parameters dictionary contains a null entry for parameter 'id' of non-nullable type 'System.Int32' for method 'Umbraco.Web.Models.ContentEditing.DataTypeDisplay GetById(Int32)' in 'Umbraco.Web.Editors.DataTypeController'. An optional parameter must be a reference type, a nullable type, or be declared as an optional parameter."},"status":400

Part of my manifest for prevalues part looks like:

prevalues: {
            fields: [
                {
                    label: "Index",
                    description: "Index to search on",
                    key: "index",
                    view: "~/App_Plugins/Similarity/views/prevalueEditor/indexes.html",
                    validation: [
                        {
                            type: "Required" 
                        }                       
                    ]
                },
                {
                    label: "Number of docs",
                    description: "Maximum number of documents to return",
                    key: "maxNo",
                    view: "number",
                    validation: [
                        {
                            type: "Required" 
                        }                       
                    ]
                }

My controller for the custom prevalue editor looks like:

angular.module("umbraco").controller("similarity.indexController",
    function ($scope, indexResource) {

        vm = this;

        indexResource.getAllIndexes().then(function(response) {

            vm.indexes = response.data;

        });
    });

In my view i have:

<div ng-controller="similarity.indexController as vm">
        <select ng-model="vm.value.index" class="umb-property-editor umb-dropdown" ng-options="index.IndexDirectory as index.IndexName for index in vm.indexes">                
            <option>choose an index</option>
        </select>       

When i save i get errors. I suspect its something todo with

ng-model="vm.value.index"

The binding on the select?

Anyone any suggestions?