title: Why is renderUI not rendering selectInput in chain of inputs
body: <p>So I have a chain of <code>selectInput()</code> that I need to render once we have the selection from the previous <code>selectInput()</code>. It seems to work fine, but the last one isn't render.  </p>
  
  <p>In my server.R file I have the following reactive expression and the renderUI expression:</p>
  
  <pre><code>  time_series_names &lt;- reactive({
      req(input$ac, input$factor_type, input$sub_type1, cancelOutput = FALSE)
      #calls an external function to get choices for selectInput...never gets
      #called and I'm not sure why...
      tryCatch({get_names_of_factors(asset_class = input$ac,
                                     factor_type = input$factor_type,
                                     sub_type = input$sub_type1)},
               error = function(e){
                 cat("Waiting on metadata to be selected...\n")
                 NULL
               })
      })
  
    output$selectTimeSeries &lt;- renderUI({
      browser()
      req(input$ac, input$factor_type, input$sub_type1, cancelOutput = FALSE)
      selectInput("var",
                  label = "Time Series",
                  choices = time_series_names(),
                  multiple = TRUE)
      })
  </code></pre>
  
  <p>So you can see the chain of three <code>input$...</code> required but I'm not sure why I can't get the <code>output$selectTimeSeries</code> to render with <code>uiOutput</code> once I have everything selected. I thought since <code>time_series_name()</code> is reactive it would trigger the <code>renderUI</code> expression? I did make sure to write <code>uiOutput("selectTimeSeries")</code> correctly in my ui.R file.</p>