No Rendering of Polygon when switching to Tab

Question

I run into a problem when working with multiple tabs. When I run the dashboard and switch to tab2, the polygon for the predefined selected radio button (Northampton) does not appear by itself. I need to press one of the buttons first for one of the polygons to appear.

So how can I get the Northampton polygon to appear immediately when I switch to tab2?

I have attached a minimalist example code below.

I think I misunderstood something with the reactive part, but I need some help to solve this.

library(shinydashboard)
library(shiny)
library(leaflet)
library(sf)
library(dplyr)

# load shapefiles 
cities <- st_read(system.file("shape/nc.shp", package="sf"))


#### Dashboard ####
header <- dashboardHeader(
  title = "example"
)

# Sidebar
sidebar <- dashboardSidebar(
  sidebarMenu(
    menuItem("Tab 1", tabName = "tab1", icon = icon("circle-info")),
    menuItem("Tab 2", tabName = "tab2", icon = icon("euro-sign"))))


# Body
body <-   dashboardBody(
  tabItems(
    tabItem(tabName = "tab1",
            h2("homescreen")
    ),
    tabItem(tabName = "tab2",
            fluidRow(
                   box(
                     title = "Map", 
                     leafletOutput("map")
                   ),
                   box(
                     title = "Choices", 
                     radioButtons("select_city", "Select the city:",
                                  c("Northampton" = "1832",
                                    "Hertford" = "1833"))
                   )
            )
    )
  )
)


# ui
ui <- dashboardPage(header, sidebar, body)


# Server
server <- function(input, output, session) {
  # basemap
  output$map <- renderLeaflet({
    leaflet(options = leafletOptions(zoomControl = TRUE, zoomSnap = 0.5, zoomDelta = 1, wheelPxPerZoomLevel = 75)) %>%  
      addProviderTiles(providers$Esri.WorldTopoMap) %>%
      setView(-77.27, 36.42, zoom = 9) %>% 
      addScaleBar(position = "topright") 
  })
  
  # determine shapefile
  reactivePolygon <- reactive({
    if (input$select_city == 1832) {
      cities[5,]
    } 
    else {
      cities[6,]
    }
  })
  
  # load shapefile to map
  observe({
    leafletProxy("map") %>%
      clearShapes() %>% 
      addPolygons(data = st_transform(reactivePolygon(), 4326)) 
  })
}

# Funktion
shinyApp(ui, server)