How to display content in nested tab menu structure (3 levels)?

Question

In order to achieve a menu item, with two sub-levels, I wrote the code below. As you can see, the text in the subMenuItems is getting displayed properly. However, when the top level tab is selected, I want to display an empty page and on the second level (menuItem inside menuItem), I want to display the text "second".

What do I need to change, in order to get there?

# shiny lib
library(shiny)
library(shinythemes)
library(shinydashboard)

# UI 
ui <- dashboardPage(
                    dashboardHeader(title = "Test"),
                    dashboardSidebar(
                      sidebarMenu(id = "tabs",
                                  menuItem("Top_Level", tabName = "first",
                                           icon = icon("chart-line"),
                                           menuItem("Second_level",
                                                    tabName = "second",
                                                    menuSubItem("Summary",
                                                                tabName = "third1",
                                                    ),
                                                    menuSubItem("Details",
                                                                tabName = "third2",
                                                              )
                                                       )   
                                                 )
                                          )
                                  ),
                    dashboardBody(
                      tabItems(
                        tabItem(tabName = "first",
                                mainPanel(width = 100, 
                                          fluidPage(fluidRow(
                                            p("")
                                          )
                                          )
                                )
                        ),
                        tabItem(tabName = "second",
                                mainPanel(width = 100, 
                                          fluidPage(fluidRow(
                                            p("second")
                                          )
                                          )
                                )
                        ),
                        tabItem(tabName = "third1",
                                mainPanel(width = 100, 
                                          fluidPage(fluidRow(
                                            p("third.1")
                                          )
                                          )
                                )
                        ),
                        tabItem(tabName = "third2",
                                mainPanel(width = 100, 
                                          fluidPage(fluidRow(
                                            p("third.2")
                                          )
                                     ) 
                                )
                           )
                     )
                )
          )

# server
server <- function(input, output, session) {
  renderText(output$first)
  renderText(output$second)
  renderText(output$third1)
  renderText(output$third2)
} 
runApp(list(ui = ui, server = server), launch.browser = TRUE)