在shinydashboard中获取带有selectInput的“参数1为空错误”

library(shiny)
library(shinydashboard)

ui <- dashboardPage(
  dashboardHeader(title = "SelectInput Demo"),
  dashboardSidebar(
    sidebarMenu(
      menuItem("Home", tabName = "home", icon = icon("home")),
      menuItem("Page1", tabName = "page1", icon = icon("info"))
    )
  ),
  dashboardBody(
    tabItems(
      tabItem(tabName = "home",
              h2("Home Page"),
              selectInput("param1", "Parameter 1", choices = c("Option 1", "Option 2"), selected = NULL),
              actionButton("btnSubmit", "Submit")
      ),
      tabItem(tabName = "page1",
              h2("Page 1"),
              verbatimTextOutput("output")
      )
    )
  )
)

server <- function(input, output, session) {
  
  observeEvent(input$btnSubmit, {
    if (is.null(input$param1)) {
      showModal(modalDialog(
        title = "Error",
        "Parameter 1 cannot be empty!",
        footer = NULL
      ))
    } else {
      # 在这里进行相关操作，参数1不为空时执行
      output$output <- renderPrint({
        paste("Selected Option:", input$param1)
      })
    }
  })
  
}

shinyApp(ui, server)