Shiny app is not working

Question

I try to build a shiny app with R but when I press RunApp the following Error Message appears:

TypeError 'undefined' is not an object (evaluating 'c.b.')

Any ideas to solve this problem? Shiny app looks like this:

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)
  )

))

server.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")
    }

  })
})