NSToolbarItemGroup has no selection and doesn't send action on click

I currently have a toolbar item group with 3 items, but clicking on any of the items doesn't do anything. Also none of the items appear to be highlighted, not even when manually setting NSToolbarItemGroup.selectedIndex. What am I missing? Setting the action property on the individual items rather than the group makes the items clickable, but still none of them appear to be selected.

class ViewController: NSViewController, NSToolbarDelegate {

    let toolbarItemIdentifier = NSToolbarItem.Identifier("group")
    let toolbarItemIdentifierItem1 = NSToolbarItem.Identifier("item1")
    let toolbarItemIdentifierItem2 = NSToolbarItem.Identifier("item2")
    let toolbarItemIdentifierItem3 = NSToolbarItem.Identifier("item3")

    override func viewDidAppear() {
        let toolbar = NSToolbar()
        toolbar.delegate = self
        view.window!.toolbar = toolbar
        view.window!.toolbarStyle = .expanded
    }
    
    func toolbarAllowedItemIdentifiers(_ toolbar: NSToolbar) -> [NSToolbarItem.Identifier] {
        return [.flexibleSpace, toolbarItemIdentifier]
    }
    
    func toolbarDefaultItemIdentifiers(_ toolbar: NSToolbar) -> [NSToolbarItem.Identifier] {
        return [.flexibleSpace, toolbarItemIdentifier, .flexibleSpace]
    }
    
    func toolbar(_ toolbar: NSToolbar, itemForItemIdentifier itemIdentifier: NSToolbarItem.Identifier, willBeInsertedIntoToolbar flag: Bool) -> NSToolbarItem? {
        switch itemIdentifier {
        case toolbarItemIdentifier:
            let item1 = NSToolbarItem(itemIdentifier: toolbarItemIdentifierItem1)
            item1.image = NSImage(named: NSImage.addTemplateName)!
            item1.label = "add"
            let item2 = NSToolbarItem(itemIdentifier: toolbarItemIdentifierItem2)
            item2.image = NSImage(named: NSImage.homeTemplateName)!
            item2.label = "home"
            let item3 = NSToolbarItem(itemIdentifier: toolbarItemIdentifierItem3)
            item3.image = NSImage(named: NSImage.pathTemplateName)!
            item3.label = "path"
            let group = NSToolbarItemGroup(itemIdentifier: itemIdentifier)
            group.subitems = [item1, item2, item3]
            group.selectionMode = .selectOne
            group.selectedIndex = 0
            group.target = self
            group.action = #selector(selectItem(_:))
            return group
        default:
            return nil
        }
    }
    
    @objc func selectItem(_ sender: Any) {
        print(0)
    }

}