`freezeReactiveValue()` causes `An unexpected error occurred during input validation` with `iv$add_rule()` and `updateSelectInput()`

[asadow]

The error is caused by:

1. adding selected = character() to updateSelectInput()
2. using choices = = character() in updateSelectInput()

ui <- fluidPage(
  selectInput("dataset", "Choose a dataset", c("pressure", "cars")),
  selectInput("column", "Choose column", character(0)),
  verbatimTextOutput("summary")
)

server <- function(input, output, session) {
  dataset <- reactive(get(input$dataset, "package:datasets"))
  
  observeEvent(input$dataset, {
    freezeReactiveValue(input, "column")
    updateSelectInput(inputId = "column", choices = names(dataset()),
                      selected = character())
  })
  
  output$summary <- renderPrint({
    summary(dataset()[[input$column]])
  })
  
  iv <- InputValidator$new()
  iv$add_rule("column", sv_optional())
  iv$enable()
  
}

shinyApp(ui, server)