Thanks for the report. I’ve confirmed that this was an Objo Studio bug, not a mistake in your project. Me.SelectedNode is valid inside the TreeView event handler.
When the Opening handler returned, the TreeView continued displaying its nodes, but the Objo VM could finalise the underlying node objects and clear their tags. This affected both object and value tags, including the Window and integer used in your example.
I’ve fixed TreeView ownership so root nodes and child nodes remain alive for as long as they are stored in the tree. They are now released correctly when removed or when the TreeView is destroyed.
The fix will be included in the next Objo Studio release: https://feedback.objo.dev/bug/1110