Linking filters in R Shiny

I have a problem creating a simple application in R Shiny. I have several filters and I need the options in the filters to change depending on the selected values at others filters. I borrowed this example (https://stackoverflow.com/a/56206037) and modified it slightly. In the input dataframe I have 2 Districts and 3 Countries. Only Germany has only District 1. So if I choose District 2 in the filter, then in the second filter I need only countries that have District 2 to be displayed.

library(shiny)
library(shinyWidgets)
library(ggplot2)
library(dplyr)


Eu1 <- data.frame(stringsAsFactors=FALSE,
                  Year = c(2018, 2017, 2016, 2015, 2018, 2017, 2016, 2015, 2018, 2017,
                           2016, 2015, 2018, 2017, 2016, 2015, 2018, 2017, 2016, 2015),
                  Districts = c("District_1", "District_1", "District_1", "District_1",
                                "District_1", "District_1", "District_1", "District_1",
                                "District_1", "District_1", "District_1", "District_1", "District_2",
                                "District_2", "District_2", "District_2", "District_2",
                                "District_2", "District_2", "District_2"),
                  Country = c("UK", "UK", "UK", "UK", "Fr", "Fr", "Fr", "Fr", "Ger", "Ger",
                              "Ger", "Ger", "Fr", "Fr", "Fr", "Fr", "UK", "UK", "UK", "UK"),
                  Amount = c(70, 63, 48, 55, 35, 41, 39, 30, 2526, 2459, 2326, 2225, 8, 6,
                             7, 14, 23, 25, 28, 29)
)


ui <- fluidPage(
  setBackgroundColor(color = c("#66e0ff", "#00a3cc", "#003d4d")),
  titlePanel("Migration"),
  sidebarLayout(
    sidebarPanel(tags$style(".well {background-color:#e6f9ff;}"),
                 sliderInput(inputId = "range",
                             label = "Chose the year range:",
                             min = 2002, max = 2020, value = c(2002,2020)),
                 selectInput(inputId = "dis",
                             label = "Chose the district",
                             choices = unique(Eu1$District)),
                 selectInput(inputId = "con",
                             label = "Chose the place of birth",
                             choices = unique(Eu1$Country))
                 #checkboxGroupInput(inputId = "con",
                  #                  label = "Chose the place of birth",
                   #                 choices = unique(Eu1$Country),
                    #                selected = unique(Eu1$Country)[1])
                 
    ),#sidebar panel
    mainPanel(plotOutput("graph1")) # closing main panel
  )#sidelayout panel
)#fluid page

server <- function(input, output){
  
  df_dat <- reactive({
    
    # make sure inputs are not NULL
    req(input$con, input$dis, input$range) 
    
    # filter data according to inputs made by the user
    df_dat <- filter(Eu1, between(Year, input$range[1], input$range[2]), Districts == input$dis, Country %in% input$con)
    
    return(df_dat)
  })
  
  # Ensures that our filter works properly
  observe(print(str(df_dat())))
  
  # create a graph 
  output$graph1 <- renderPlot({
    
    # use to debug:
    # browser()
    
    # make sure filtered data is not NULL
    req(df_dat())
    
    # plot filtered data
    ggplot(df_dat(), aes(x = Year, y = Amount)) +
      geom_line(aes(colour = Country)) +
      geom_point()
  })
  
}

shinyApp(ui = ui, server = server)

Problem in output:

You can use updateSelectInput to update it. Try this

server <- function(input, output, session){
  
  df_dat <- reactive({
    
    # make sure inputs are not NULL
    req(input$con, input$dis, input$range) 
    
    # filter data according to inputs made by the user
    df_dat <- filter(Eu1, between(Year, input$range[1], input$range[2]), Districts == input$dis, Country %in% input$con)
    
    return(df_dat)
  })
  
  observeEvent(input$dis,{
    Eu <- filter(Eu1, between(Year, input$range[1], input$range[2]), Districts == input$dis)
    updateSelectInput(session,"con", choices=unique(Eu$Country))
  })
  
  # Ensures that our filter works properly
  observe(print(str(df_dat())))
  
  # create a graph 
  output$graph1 <- renderPlot({
    
    # use to debug:
    # browser()
    
    # make sure filtered data is not NULL
    req(df_dat())
    
    # plot filtered data
    ggplot(df_dat(), aes(x = Year, y = Amount)) +
      geom_line(aes(colour = Country)) +
      geom_point()
  })
  
}