Gadget gets error at startup

app.R

# This code produces an error at startup

require(miniUI)
require(shiny)

ff <- function () 
{

  ui <- miniPage(
    gadgetTitleBar("Variable Profiler"), 
    miniContentPanel(
          selectInput("data", "Data frame", choices = c('iris','mtcars')), 
          selectInput("colSelect", "Columns", choices = NULL),
          plotOutput("plot")
    ))

  server <- function(input, output, session) {

    data <- reactive({
      get(input$data)
    })
    
    vec <- reactive({
      data()[[input$colSelect]]
    })
    
    observe({
      updateSelectInput(session, "colSelect", choices = names(data()))
    })
    
    output$plot <- renderPlot({
      #validate(need(is.null(input$colSelect), 'NULL Column Selected')) # does not solve error
      hist(vec())
    })

    observeEvent(input$done, {
      stopApp(out)
    })
    
  }
  
  runGadget(ui, server)
  
}