TreeViewItemsIssue.kt

    private val treeView: TreeView<String> by fxid("MenuTreeView")
    var rootTreeNode = TreeItem("Root")

    private val balanceSettings = TreeItem<String>("Balance") // Expandable
    private val serverBalanceSettings = TreeItem<String>("Server") // Expandable
    private val regionsQtySettings = TreeItem<String>("Regions-Qty")
    private val applicationSettings = TreeItem<String>("Application")
    
    init {
        balanceSettings.children.add(TreeItem<String>("Bronze"))
        balanceSettings.children.add(TreeItem<String>("Silver"))
        balanceSettings.children.add(TreeItem<String>("Golden"))
        balanceSettings.children.add(TreeItem<String>("Platinum"))
        balanceSettings.children.add(TreeItem<String>("Niobium"))
        balanceSettings.isExpanded = true

        serverBalanceSettings.children.add(TreeItem<String>("Global"))
        serverBalanceSettings.isExpanded = true

        rootTreeNode.children.add(balanceSettings)
        rootTreeNode.children.add(serverBalanceSettings)
        rootTreeNode.children.add(regionsQtySettings)
        rootTreeNode.children.add(applicationSettings)

        treeView.root = rootTreeNode

        treeView.setOnMouseClicked {
            with (treeView.selectionModel.selectedItem) {
                isExpanded = !isExpanded
            }
        }
    }