Trying to get my plugin working in Umbraco 8.1

Cant edit the fields in the back office. Get an error in the concole "TypeError: Cannot create property 'HeadingText' on string ''

What are the changes between 7 and 8 as the following cod works in 7

.js

angular.module('umbraco').controller('IBD_HeaderController', function ($scope) {

$scope.model.value.Tag = $scope.model.value.Tag || "default value";
$scope.model.value.HeadingText = $scope.model.value.HeadingText || "default value";

console.log($scope.model);

$scope.SetTag = function () {
    $scope.model.value.Tag = $scope.value.Tag
};

});

.html

<div class="IBD_CMS" ng-controller="IBD_HeaderController">
    <div>
        <input class="headingTextBox" type="text" ng-model="model.value.HeadingText" ng-change="SetHeading()">
    </div>
    <div class="HeadingTag">
        <label class="TagOption">
            <input type="radio" name="HeadingTag" value="h1" ng-model="model.value.Tag" ng-click="SetTag()">
            <span>H1</span>
        </label>
        <label class="TagOption">
            <input type="radio" name="HeadingTag" value="h2" ng-model="model.value.Tag" ng-click="SetTag()">
            <span>H2</span>
        </label>
        <label class="TagOption">
            <input type="radio" name="HeadingTag" value="h3" ng-model="model.value.Tag" ng-click="SetTag()">
            <span>H3</span>
        </label>
        <label class="TagOption">
            <input type="radio" name="HeadingTag" value="h4" ng-model="model.value.Tag" ng-click="SetTag()">
            <span>H4</span>
        </label>
        <label class="TagOption">
            <input type="radio" name="HeadingTag" value="h5" ng-model="model.value.Tag" ng-click="SetTag()">
            <span>H5</span>
        </label>
        <label class="TagOption">
            <input type="radio" name="HeadingTag" value="h6" ng-model="model.value.Tag" ng-click="SetTag()">
            <span>H6</span>
        </label>
    </div>
</div>

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