Upload a .xlsx file to Shiny and load in all the tabs as a list

Question

I want to upload an .xlsx file to Shiny and load all the tabs in as a list. In the example, I am expecting the object input_file to be a list of dataframes which are the tabs from the .xlsx file. The example is selecting the first dataframe and then putting it in a flextable. The code doesnt work, I get the error message, object of type 'closure' is not subsettable Can anyone suggest anything to fix this?

library(shiny)
library(readxl)
library(flextable)
library(tidyverse)

ui<-shinyUI(fluidPage(
  titlePanel("Use readxl"),
  sidebarLayout(
    sidebarPanel(
      fileInput('file1', 'Choose xlsx file',
                accept = c(".xlsx")
      )
    ),
    mainPanel(
      
      htmlOutput ("ftable2")
     
    )
  )
)

)


server <- function(input, output){
  input_file<-reactive({
    if (is.null(input$file1)) {
      return("")
    }
    
    sapply(readxl::excel_sheets(input$file1$datapath), simplify = F, USE.NAMES = T,
           function(X) readxl::read_excel(input$file1$datapath, sheet = X))
    
    
  })
  
  output$ftable2<- renderUI( { 
    req(input_file())
    
    input_file[[1]]%>%
      head(10)%>%
      flextable()%>%
      htmltools_value()
  })}
shinyApp(ui, server)