title: Shiny renderUI selectInput returned NULL
body: <p>I am trying to use a reactivity model with one input affecting several outputs as describe in the shiny cheat sheet.
  I need to use renderUI because the choices list is rendered dynamically (not shown in the example)
  However, during initialization selectInput returns NULL rather than the default value. After that first NULL value the input works as expected. I am new to shiny and might be doing something wrong. </p>
  
  <p>UPDATE: other controls unexpectedly returned not only NULL, but also NA after initialization.  </p>
  
  <p>See code below. See the console output, the first input returning NULL.</p>
  
  <pre><code>Listening on http://127.0.0.1:6211
   NULL
   chr "1"
   chr "2"
   chr "1"
  
  
  
  
  library(shiny)
  
  runApp(list(
  
    ui = bootstrapPage(
      fluidPage( uiOutput('ui.A')   )
    ),
  
  
    server = function(input, output){
  
      output$ui.A = renderUI({
        selectInput("A", label = h4("input A"), 
                    choices = list(A_1=1, A_2=2), 
                    selected = 1)
      })
  
      A.r &lt;- reactive({input$A })
  
      observe({ 
  
        A &lt;- A.r()
        str(A)
  
      })
  
    }))
  </code></pre>