title: Shiny renderUI not Selectinput stopped updating on its own
body: <p>I am a newbie in Shiny. I am working on a dashboard. I am using renderUI to get some input. For 2 days, the code was working fine but suddenly today it started giving me issues. Any help would be much appreciated.</p>
  
  <p>ui code:</p>
  
  <pre><code>ui &lt;- fluidPage(
  column(width =6,
         box(width = NULL, status = "warning",solidHeader = TRUE,
              title = "Topics Level",
              selectInput(inputId = "Vis_level_select", 
                          label = "",
                          selected = "Main Topics",
                          choices = c("Main Topics","Sub-Topics")
                         )
                     )
              ),
  
  ## Topic Number Output
  column(width = 6,
         box(width = NULL, status = "warning",solidHeader = TRUE,
             title = "Select the topic for which Sub Topics are to be shown",
             uiOutput("topic_no_select")
             )
        ))
  </code></pre>
  
  <p>server code:</p>
  
  <pre><code>server &lt;- function(input, output){
  ## Topic number output
  output$topic_no_select &lt;- renderUI({
    if(input$Vis_level_select == "Main Topics"){
    selectInput(inputId = "main_topic_no", label = "", choices = "N/A", selected = "N/A")
  } else{
    selectInput(inputId = "main_topic_no", label = "", choices = c(1:10), selected = 1)
  }
  })
  
  ## LDAvis output
  output$LDAvis_output &lt;- renderVis({
    if(input$Vis_level_select == "Sub-Topics"){
    viz_file &lt;- paste0("Viz_Sub_topics_Main_Topic_",input$main_topic_no)
  } else if(input$Vis_level_select == "Main Topics"){
    viz_file &lt;- "Viz_Sub_topics_Main_Topics"
  }
    get(viz_file)
  })
  }
  </code></pre>
  
  <p>The if statement in <code>LDAvis</code> output gives me blank and it gives me an error saying that <code>Viz_Sub_topics_Main_Topic_</code> not found. Once I change the value of <code>input$Vis_level_select</code> then the first part of <code>LDAvis</code> works but still the second part is not working. I am unable to understand why this is happening.</p>
  
  <p>Please let me know if this is just a bug that will go away on restarting.</p>