title: shinyglide conditional buttons not working on first screen
body: <p>I'm developing a shiny app using the great shinyglide package to create a modal for the user to enter some information. The problem is that on the first screen I would need to create a conditional so that the user must enter a value in order to proceed. Unfortunately, I can't use the <code>next_condition</code> command on the first screen, seems to only work from the second screen.  </p>
  
  <p>When I click the "Next" button and then return to the first screen using the "Back" button, the condition works and I can't go forward until I click in at least one option, but the first time it does not work that way.  </p>
  
  <p>Is this behavior normal? Is it possible to set another type of condition in this case?  </p>
  
  <p>I made a small example to show the problem:</p>
  
  <pre><code>library(shiny)
  library(shinyglide)
  
  ui &lt;- fixedPage(
    titlePanel("shinyglide modal example"),
    sidebarLayout(
      sidebarPanel(
        p('Hello World')
        ),
      mainPanel()
      )
    )
  
  server &lt;- function(input, output, session) {
  
    modal_controls &lt;- glideControls(
  
      list(prevButton(),
           firstButton(class = "btn btn-danger",`data-dismiss`="modal","No, thanks !")),
  
      list(nextButton(),
           lastButton(class = "btn btn-success",`data-dismiss`="modal","Done"))
  
      )
  
    glide_modal &lt;- modalDialog(
  
      title = "Startup assistant",easyClose = FALSE,footer = NULL,
  
      glide(custom_controls = modal_controls,
  
            screen(next_condition="input.options.length &gt; 0",
                   p("First, please select an option"),
                   checkboxGroupInput("options", "Options", choices=c('a','b','c'),selected=NULL)
                   ),
  
            screen(p("Next, please select a number"),
                   numericInput("number", "Number", value = 1, min = 0)
                   ),
  
            screen(p("Thanks, we're all set !"))
  
            )
  
      )
  
    showModal(glide_modal)
  
  }
  
  shinyApp(ui, server)
  </code></pre>