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我尝试使用 R 构建一个闪亮的应用程序，但是当我按下 RunApp 时出现以下错误消息：

TypeError 'undefined' 不是对象（评估 'cb'）

有什么想法可以解决这个问题吗？闪亮的应用程序看起来像这样：

ui.R:

library(shiny)
shinyUI(bootstrapPage(

  selectInput(inputId = "n_breaks",
              label = "Number of bins in histogram (approximate):",
              choices = c(10, 20, 35, 50),
              selected = 20),

  checkboxInput(inputId = "individual_obs",
                label = strong("Show individual observations"),
                value = FALSE),

  checkboxInput(inputId = "density",
                label = strong("Show density estimate"),
                value = FALSE),

  plotOutput(outputId = "main_plot", height = "300px"),

  # Display this only if the density is shown
  conditionalPanel(condition = "input.density == true",
                   sliderInput(inputId = "bw_adjust",
                               label = "Bandwidth adjustment:",
                               min = 0.2, max = 2, value = 1, step = 0.2)
  )

))

服务器.R：

library(shiny)
shinyServer(function(input, output) {

  output$main_plot <- renderPlot({

    hist(faithful$eruptions,
         probability = TRUE,
         breaks = as.numeric(input$n_breaks),
         xlab = "Duration (minutes)",
         main = "Geyser eruption duration")

    if (input$individual_obs) {
      rug(faithful$eruptions)
    }

    if (input$density) {
      dens <- density(faithful$eruptions,
                      adjust = input$bw_adjust)
      lines(dens, col = "blue")
    }

  })
})
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