library(shiny)

shinyApp(
  ui = basicPage(
    actionButton("show", "Show modal dialog"),
    verbatimTextOutput("print")
  ),

  server = function(input, output) {

    vals <- reactiveValues(txt = NULL)

    # Create modal
    popupModal <- function(failed = FALSE) {
      modalDialog(
        textInput("txt", "Write something"),
        if (failed)
          div(tags$b("You did not input anything", style = "color: red;")),

        footer = tagList(
          modalButton("Cancel"),
          actionButton("ok", "OK")
        )
      )
    }

    # Show modal when button is clicked.
    observeEvent(input$show, {
      showModal(popupModal())
    })

    observeEvent(input$ok, {

      if (!is.null(input$txt) && nzchar(input$txt)) {
        vals$txt <- input$txt
        removeModal()
      } else {
        showModal(popupModal(failed = TRUE))
      }
    })

    # Print inputted text
    output$print <- renderPrint({
      if (is.null(vals$txt))
        "No data selected"
      else
        vals$txt
    })
  }
)