?tmapOutput - example does not work

[Nowosad]

An example in the ?tmapOutput help file returns an error:

library(tmap)
library(shiny)
data(World)
world_vars <- setdiff(names(World), c("iso_a3", "name", "sovereignt", "geometry"))


ui <- fluidPage(
  tmapOutput("map"),
  selectInput("var", "Variable", world_vars)
)

server <- function(input, output, session) {
  output$map <- renderTmap({
    tm_shape(World) +
      tm_polygons(world_vars[1], zindex = 401)
  })
  
  observe({
    var <- input$var
    tmapProxy("map", session, {
      tm_remove_layer(401) +
        tm_shape(World) +
        tm_polygons(var, zindex = 401)
    })
  })
}	


app <- shinyApp(ui, server)
if (interactive()) app

Warning: Error in if: argument is of length zero
  [No stack trace available]