🐛 `TreeViewItem` can't be opened programatically

[bdlukaa]

Describe the bug
When setting TreeViewItem.expanded to true, only the icon is changed. The item children does not show up!

To Reproduce
Steps to reproduce the behavior:

1. Create a final item = TreeViewItem();
2. Call item.expanded = true;
3. Item is not expanded

Expected behavior
The item to be expanded

Additional context
n/a