To reproduce:
- Create a property store and add some items to it.
- Start the project and click an item.

Workaround: - 
Add RadSortOrderAttribute to each item:

for (int i = 0; i < propStore.Count; i++)
                {
                    PropertyStoreItem item = propStore[i];
                    item.Attributes.Add(new RadSortOrderAttribute(i));
                }