### 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
List<StringName> names;
	Ref<Theme> old_snapshot = edited_theme->duplicate();
	Ref<Theme> new_snapshot = edited_theme->duplicate();
	EditorUndoRedoManager *ur = EditorUndoRedoManager::get_singleton();
	ur->create_action(TTR("Remove Class Items From Theme"));
	for (int dt = 0; dt < Theme::DATA_TYPE_MAX; dt++) {
		Theme::DataType data_type = (Theme::DataType)dt;
		names.clear();
		ThemeDB::get_singleton()->get_default_theme()->get_theme_item_list(data_type, edited_item_type, &names);
		for (const StringName &E : names) {
			if (new_snapshot->has_theme_item_nocheck(data_type, E, edited_item_type)) {
				new_snapshot->clear_theme_item(data_type, E, edited_item_type);
				if (dt == Theme::DATA_TYPE_STYLEBOX && theme_type_editor->is_stylebox_pinned(edited_theme->get_stylebox(E, edited_item_type))) {
					ur->add_do_method(theme_type_editor, "_unpin_leading_stylebox");
					ur->add_undo_method(theme_type_editor, "_pin_leading_stylebox", E, edited_theme->get_stylebox(E, edited_item_type));
				}
			}
		}
	}
	ur->add_do_method(*edited_theme, "clear");
	ur->add_do_method(*edited_theme, "merge_with", new_snapshot);
	ur->add_undo_method(*edited_theme, "merge_with", old_snapshot);
	ur->add_do_method(this, "_update_edit_item_tree", edited_item_type);
	ur->add_undo_method(this, "_update_edit_item_tree", edited_item_type);
	ur->commit_action();
}
void ThemeItemEditorDialog::_remove_custom_items() {