So I have a chain of selectInput() that I need to render once we have the selection from the previous selectInput(). It seems to work fine, but the last one isn't render.  
In my server.R file I have the following reactive expression and the renderUI expression:
  time_series_names <- 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$selectTimeSeries <- renderUI({
    browser()
    req(input$ac, input$factor_type, input$sub_type1, cancelOutput = FALSE)
    selectInput("var",
                label = "Time Series",
                choices = time_series_names(),
                multiple = TRUE)
    })
So you can see the chain of three input$... required but I'm not sure why I can't get the output$selectTimeSeries to render with uiOutput once I have everything selected. I thought since time_series_name() is reactive it would trigger the renderUI expression? I did make sure to write uiOutput("selectTimeSeries") correctly in my ui.R file.
