debugging shiny

I'm afraid I need more help debugging my shiny code.

I added browser() to a renderUI statement as someone suggested. That works fine when I create my base.list() from lists. However, it doesn't work when I create base.list() from an uploaded file. I need to work with updated files like the one I copied below the following R code.

Thanks for any help you can give me on this.

library(tidyverse)
library(shiny)
library(lubridate)
library(ggplot2)


ui <- fluidPage(

  titlePanel("What if? Trend"),
  
  sidebarLayout(

    sidebarPanel(
      fileInput("upload.file", "Upload trend data file",
        multiple = FALSE,
        accept = ".csv"),
      uiOutput("base.date.output"),
      uiOutput("launch.date.output"),
    ),
    
    mainPanel(
    )
  )
)

server <- function(input, output, session) {
  
  # upload trend data file ----
  t.input <- eventReactive(input$upload.file, {
   read_csv(input$upload.file$datapath)
  })
  #----
  
  # create base.list from uploaded trend data file ---
  base.list <-reactive(
    {
      req(t.input())
      as.list(t.input()[,1])
    }
  )
  #----
  
  # # create base.list from lists ----
  # list1 <- list(1900,1910,1920,1930,
  #               1940,1950,1960,1970
  #               ,1980,1990,2000,2010)
  # list1 <- as.data.frame(list1)
  # list1 <- t(list1)
  # list2 <- list(21.112,27.881,44.051,70.278,
  #               86.942,136.395,256.782,415.387,
  #               483.024,545.837,665.865,691.893)
  # list2 <- as.data.frame(list2)
  # list2 <- t(list2)
  # t.input <- reactive(cbind(list1,list2))
# 
#   base.list <-reactive(
#     {
#       req(t.input())
#       setNames( as.list(t.input()[,1]),
#                 NULL)
#     }
#   )
#   #----
  
   launch.list <- reactive(
        base.list()
   )

  # Specify parameters ----
  
  output$base.date.output <- renderUI({
    browser()
    selectInput("base.date",
                label = "Select base date",
                choices = base.list(),
                selected = "")
  })
  
  output$launch.date.output <- renderUI({
    selectInput("launch.date",
                label = "Select launch date",
                choices = launch.list(),
                selected = "")
  })
  
}

shinyApp(ui, server)

date popln
1900 21.112
1910 27.881
1920 44.051
1930 70.278
1940 86.942
1950 136.395
1960 256.782
1970 415.387
1980 483.024
1990 545.837
2000 665.865
2010 691.893