Shiny example: Diamonds Explorer

server.R

library(shiny)
library(ggplot2)

function(input, output) {

  dataset <- reactive({
    diamonds[sample(nrow(diamonds), input$sampleSize),]
  })

  output$plot <- renderPlot({
    
    p <- ggplot(dataset(), aes_string(x=input$x, y=input$y)) + geom_point()
    
    if (input$color != 'None')
      p <- p + aes_string(color=input$color)
    
    facets <- paste(input$facet_row, '~', input$facet_col)
    if (facets != '. ~ .')
      p <- p + facet_grid(facets)
    
    if (input$jitter)
      p <- p + geom_jitter()
    if (input$smooth)
      p <- p + geom_smooth()
    
    print(p)
    
  }, height=700)
  
}

ui.R

library(shiny)
library(ggplot2)

dataset <- diamonds

pageWithSidebar(

  headerPanel("Diamonds Explorer"),
  
  sidebarPanel(

    sliderInput('sampleSize', 'Sample Size', min=1, max=nrow(dataset),
                value=min(1000, nrow(dataset)), step=500, round=0),
    
    selectInput('x', 'X', names(dataset)),
    selectInput('y', 'Y', names(dataset), names(dataset)[[2]]),
    selectInput('color', 'Color', c('None', names(dataset))),
    
    checkboxInput('jitter', 'Jitter'),
    checkboxInput('smooth', 'Smooth'),
    
    selectInput('facet_row', 'Facet Row', c(None='.', names(dataset))),
    selectInput('facet_col', 'Facet Column', c(None='.', names(dataset)))
  ),

  mainPanel(
    plotOutput('plot')
  )
)

I am getting an error:

Warning: Error in as.character: cannot coerce type 'environment' to vector of type 'character'
Stack trace (innermost first):
    4: <Anonymous>
    3: do.call
    2: print.shiny.appobj
    1: <Promise>
Error : cannot coerce type 'environment' to vector of type 'character'