title: mainPanel argument missing in ui.r file using R's Shiny package
body: <p>I am running <code>Shiny</code> in R to create a GUI, and I receive the following error: <code>ERROR: argument "mainPanel" is missing, with no default</code></p>
  
  <p>However, as you can see from my code below in the ui.r file, I do have <code>mainPanel</code> included at the bottom, which should be getting successfully passed to <code>sidebarLayout</code>:</p>
  
  <pre><code>require(shiny)
  
  shinyUI(fluidPage(
  titlePanel("Approach"),
  sidebarLayout(
      sidebarPanel(
          fileInput('file1', 'Choose file to upload',
                  accept = c(
                      'text/csv',
                      'text/comma-separated-values',
                      'text/tab-separated-values',
                      'text/plain',
                      '.csv',
                      '.tsv'
                  )
          )
  
      )
  ),
      mainPanel(
          plotOutput("plot")
      )
  
  
  
  )
  )
  </code></pre>
  
  <p>I have also made sure to include a comma before <code>mainPanel</code>, as suggested in <a href="https://stackoverflow.com/questions/22626412/error-in-rshiny-ui-r-argument-missing">Error in RShiny ui.r argument missing</a> </p>
  
  <p>Any suggestions are greatly appreciated.</p>