title: Radio button on shiny app error
body: <p>I have made a shiny code that would take input from user and save answer in x=excel file. When I add the radio button code line(commented) in question list I get error. Why is the error and what can be done to resolve it?</p>
  
  <pre><code>install.packages("devtools")
  install.packages("digest")
  install.packages("shiny")
  devtools::install_github("daattali/shinyforms")
  library(shiny)
  library(shinyforms)
  
  questions &lt;- list(
    list(id = "name", type = "text", title = "Name", mandatory = TRUE),
    list(id = "age", type = "numeric", title = "Age", mandatory = TRUE),
    list(id = "Income", title = "Your average annual income from all sources before tax (e.g. Pension, employment, investments, etc.) is", type = "text", mandatory = TRUE),
    list(id = "saving", type = "text", title = "You regularly save the following percentage of your income for special expenditures, such as education, mortgage lump sum repayments, retirement, etc", mandatory = TRUE),
    list(id = "continue", title = "You expect your current income level (at a minimum) to continue for the:", type = "text", mandatory = TRUE),
    list(id = "status", type = "text", title = "You would describe your financial situation as being:", mandatory = TRUE),
  #radioButtons("gender", label = ("select the Gender"),choices = list("Male" = 1, "Female" = 2),selected = 1),
    list(id = "correct", type = "checkbox", title = "I have given correct answers")
     )
  formInfo &lt;- list(
    id = "basicinfo",
    questions = questions,
  
    storage = list(
      # Right now, only flat file storage is supported
      type = STORAGE_TYPES$FLATFILE,
      # The path where responses are stored
      path = "D:\\CP\\responses\\"
    )
  )
  
  
  ui &lt;- fluidPage(
    #titlePanel("Registation Form"),
    formUI(formInfo)
  )
  
  server &lt;- function(input, output, session) {
    formServer(formInfo)
  }
  
  shinyApp(ui = ui, server = server)
  </code></pre>
  
  <p>Error: I get the below error when I run the ui &lt;- fluidPage</p>
  
  <p>Error in if (question$id %in% fieldsMandatory) { :    argument is of
  length zero</p>