consider the following example
Select an item, let's say "avalon", by clicking on it. Notice the text under the text field informing you about the currently selected item.
Now apply a filter, let's say "123". The tree nodes disappear and selected item changes to null which is expected I believe.
Now remove the filter completely. The "Avalon" tree node appears again and is visually highlighted/selected but the getSelectedItem() tree method still returns null. I believe this is a bug.
Either remove also the visual selection or make sure the getSelectedItem() method always returns the highlighted tree item - the second would be my preferred solution.