我试图用actionButton触发的selectInput更新一个React式表达式，但是没有成功。下面是（几乎）最简单的示例代码：

library(tidyverse)
library(shiny)
library(DT)

data <- tibble(ID = 1:9,
               x = c(5, 4, 3, 4, 5, 7, 4, 2, 5),
               min = c(NA, NA, -1, NA, NA, NA, NA, -1, NA),
               rating = NA_integer_)

ui <- fluidPage(
  DTOutput("tbl", width = "100%"),
  hr(),
  fluidRow(
    column(4, selectInput(inputId = "min", label = "Choose min", choices = 1)),
    column(4, selectInput(inputId = "rating", label = "Please rate",
                          choices = c("Choose one", "1: Yes" = "1", "2: No" = "2"))),
    column(4, fluidRow(
      column(12, tags$div(HTML("<p style = \"margin-bottom: 5px;\"><strong>Submit</strong></p>"))),
      column(12, actionButton("submit", "Submit rating and save to data"))
    )))
)

server <- function(input, output, session) {
  mins <- reactive({
    data %>% filter(min == -1) %>% pull(ID)
  })
  observeEvent(mins(), {
    updateSelectInput(session, inputId = "min", choices = mins())
  })
  mins_table <- reactive({
    data %>% filter(ID %in% mins())
  })
  tbl <- reactive({
    DT::datatable(mins_table(),
                  caption = "Min to rate",
                  rownames = FALSE,
                  options = list(paging = FALSE, 
                                 scrollX = FALSE, 
                                 searching = FALSE, 
                                 ordering = FALSE, 
                                 lengthChange = FALSE)) %>%
                    formatStyle("ID", target = "row", fontWeight = styleEqual(as.integer(input$min), "bold"))
  })
  output$tbl <- renderDT({
    tbl()
  })
  observeEvent(input$submit, {
    tmp <- which(mins() == input$min)
    # write rating to mins_table (to show rating in app) --> doesn't work:
    ################ Error occurs in the following line 
    mins_table()$rating[tmp] <<- as.integer(input$rating) # Error in <<-: invalid (NULL) left side of assignment
    # write rating to data and save file locally (overwrite) --> works fine
    data$rating[data$ID == input$min] <<- as.integer(input$rating)
    saveRDS(data, file = "output/data2.rds")
    # go to next min
    updateSelectInput(session, inputId = "min", selected = mins()[tmp + 1])
  })
}

shinyApp(ui, server)

错误发生在observeEvent中。如有任何帮助，将不胜感激。