Hello Nigel Shaw,
The targetTopic is null because e.TargetDropItem is the dynamically generated container for your business object. Therefore you need to cast it to the type of the container (RadTreeViewItem) like this:
RadTreeViewItem container = e.TargetDropItem as RadTreeViewItem;
Afterwards, you can access the business object itself via the container.Item property:
Model.Topic targetTopic = container.Item as Model.Topic;
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