Tree openOnChecked not working

[alterwiel]

I have the following code, which all works, except for the openOnChecked property of the tree. Are there any obvious mistakes here?

var layers = {};
layers.label = 'name';
layers.identifier = 'name';
layers.items = [];
layers.items.push({ id: -1, type: 'root', label: 'Root', name: 'Root' });

 // data is result of a call to a REST service
array.forEach(data.layers, function (item) {
   if (item.parentLayerId < 0) {
       layers.items.push({ id: item.id, type: 'parent', label: item.name, name: item.name, parent: -1, subLayerIds: item.subLayerIds });
   }
   else {
       layers.items.push({ id: item.id, type: 'child', label: item.name, name: item.name, parent: item.parentLayerId, subLayerIds: item.subLayerIds }); 
   }
});

var store = Observable(new Memory({ data: layers.items }));
that._model = new TreeStoreModel({
   store: store,
   query: {
        id: -1
   },
   rootLabel: "Layers",
   checkedRoot: true
});

var tree = new Tree({
    model: that._model,
    id: "MenuTree",
    openOnChecked: true
}, that.checkboxTreeDiv);