title: observeEvent() and reactiveValues() - How to create good placeholder
body: <p>I create an app where I have two <code>actionButtons</code></p>
  
  <pre><code>ui.R
          h4("First | Second "),
          actionButton(inputId = "first", 
                       label = "First"),
          actionButton(inputId = "second", 
                       label = "Second"),
  </code></pre>
  
  <p>Thats my <code>server.R</code></p>
  
  <pre><code>#create the datasetbased on the choice of "First" or "Second"
  myDataSet &lt;- reactiveValues(data = NULL)
  
  
  #observer to see what is clicked. Decision is send to "reactiveValues"
  observeEvent(input$first, {
          myDataSet$data &lt;- dataframe1
  })
  
  
  #observer to see what is clicked. Decision is send to "reactiveValues"
  observeEvent(input$second, {
          myDataSet$data &lt;- dataframe2
  })
  </code></pre>
  
  <p>Depending on the input coming from the <code>actionButton</code> I want to store <code>dataframe1</code> or <code>dataframe2</code> in <code>myDataSet</code>.
  As written here: Is that the way in <code>reactiveValues()</code> to create the variable which will be holding my <code>dataframes</code>? Note, the extend (columns/rows) of my <code>dataframes</code> send in the <code>observeEvent()</code> varies.</p>