REL-1301

We now must check for `null` for the model in the `propertyChange` event, since the `RefreshAction`, in order to force data objects to all update when the model changes, performs a:
`setModel(null); setModel(newModel);`
This was causing issues with `evt.getNewValue()` being `null` from the first `setModel` call, which caused this UI abomination of a tree to be rebuilt and some of the node statuses to be reset, which is not what we want, so property change with `null` must be ignored.