shiny app works locally but not on shinyapps.io

I have browsed on SO and the R community, it looks like this error may result from vary reasons, so I still post my problem here, please give me some suggestions. Thank you very much.

Here is the code that works locally:

library(shiny)

ui=fluidPage(
  titlePanel('hello shiny'),
  sidebarPanel(
    textInput('ID',label='ID:'),actionButton('goButton',label='submit')
    ),
  mainPanel(imageOutput('out'))
)

server <- function(input, output,session) {
  fullnames=list.files(path = "/Users/u/myapp/www", pattern = ".png");
  reactive({
            input$ID=gsub(pattern = '\\.png','',fullnames)
            }
           );
  output$out=renderImage(
    {
     filenames=normalizePath(file.path('/Users/u/myapp/www',paste(input$ID,'.png',sep='')));
     list(src=filenames)
    },
    deleteFile = F
     
  )
}

shinyApp(ui,server)

however, there is a warning:

Listening on http://127.0.0.1:7453
Warning in normalizePath(file.path("/Users/u/myapp/www", paste(input$ID,  :
  path[1]="/Users/ya/myapp/www/.png": No such file or directory

I guess it is the input that caused this error, but the similar warning on the server log shows that there is an ID '20191704465.png' in the code (the ID I fill in in the textInput widget):

2021-08-12T13:32:22.863287+00:00 shinyapps[4485062]: Listening on http://127.0.0.1:38432
2021-08-12T13:32:26.831219+00:00 shinyapps[4485062]:   path[1]="/Users/u/myapp/www/.png": No such file or directory
2021-08-12T13:32:26.831217+00:00 shinyapps[4485062]: Warning in normalizePath(file.path("/Users/u/myapp/www", paste(input$ID,  :
2021-08-12T13:32:31.471638+00:00 shinyapps[4485062]:   path[1]="/Users/u/myapp/www/20191704465.png": No such file or directory

but somehow the server still shows:

path[1]="/Users/u/myapp/www/20191704465.png": No such file or directory

What went wrong with the code?

While it can run locally with

path = "/Users/u/myapp/www",

it is best to specify relative path while running on shiny server as

path = "./www"