checkbox not selected in a shiny app - r

Question

I need to prepare a shiny app for a school project.
This is a link of what it is supposed to look like
https://yuvaln.shinyapps.io/olympics/
If you look at the app you see there is a checkbox named medals.When you
open the app they are all selected but in the event the user decides to uncheck them all there should be a small error and no graph should be drawn.
I am having trouble getting to this, when I uncheck all the boxes in my app
it draws an empty drawing
This is the important part of the code:
fluidRow(
column(3,checkboxGroupInput("Medals", label = strong("Medals"),
choices = list("Total" = "TOTAL", "Gold" = 'GOLD',
"Silver" = 'SILVER','Bronze'='BRONZE'),
selected = c('TOTAL','GOLD','SILVER','BRONZE')))),
fluidRow(
mainPanel(plotOutput('coolplot'),width = '40%'))
)
)
server <- function(input, output){output$coolplot<-renderPlot(plot.medals2(input$country,
input$Startingyear,input$Endingyear,input$Medals))}
shinyApp(ui = ui, server = server)
I am using a function plot.medals2 that gets a vector of medals ,start year, ending year, country and returns a drawing of the graph.

Answer 1

Since you didn't post the complete code, I have recreated an example using the Iris data set. I guess the code below answers your question...
library(shiny)
library(ggplot2)
library(dplyr)
ui <- shinyUI(fluidPage(
# Application title
titlePanel("Checkbox example"),
fluidRow(
column(3,checkboxGroupInput("example", label = strong("Species"),
choices = levels(iris$Species),
selected = levels(iris$Species)))),
fluidRow(
mainPanel(plotOutput('coolplot'),width = '40%'))
))
server <- shinyServer(function(input, output) {
irisSubset <- reactive({
validate(
need(input$example != "", 'Please choose at least one feature.')
)
filter(iris, Species %in% input$example)
})
output$coolplot<-renderPlot({
gg <- ggplot(irisSubset(), aes(x = Species, y = Sepal.Length))
gg <- gg + geom_boxplot()
print(gg)
})
})
# Run the application
shinyApp(ui = ui, server = server)