### Godot version

v4.0.beta.custom_build [eaf306e0b]

### System information

Windows 10

### Issue description

Selecting a `Window` node in the Editor's SceneTree results in the message
```
editor/editor_data.cpp:1058 - Parameter "p_node" is null.
```
and the Inspector shows empty content instead of the Window's properties.

I would expect, that clicking on a node in the SceneTree lets me edit the properties of the node.

Bisecting shows that this behavior got introduced in #71770

### Steps to reproduce

1. Create a Window node
2. Click on the Window node in the Editor's SceneTree

### Minimal reproduction project

N/A
TreeItem *item = Object::cast_to<TreeItem>(p_obj);
	
	if (item && !item->get_first_child()) {
		_store_selected_item(item);
	}
}
void ThemeItemImportTree::_select_all_subitems(TreeItem *p_root_item, bool p_select_with_data) {