title: Reactives: invalid (NULL) left side of assignment
body: <p>I'm trying to extract an output of a personal function called tests.stats()</p>
  
  <p>I make a function with <code>return</code> statement:</p>
  
  <pre><code>   return(c(list.test.1, list.test.2 ,list.test.3, list.test.4))
  </code></pre>
  
  <p>Where each <code>list.test.&lt;i&gt;</code> is a <code>list</code> object.</p>
  
  <p>In Shiny Server:</p>
  
  <pre><code> outlist() &lt;- reactive({
     if(is.null(input$datafile)){
       return()
     }
     if(is.null(input$varinteret)
        &amp;&amp;is.null(input$vartemps)
        &amp;&amp;is.null(input$varsujet)
        &amp;&amp;is.null(input$varprod) 
        &amp;&amp;is.null(input$apparie)
        &amp;&amp;is.null(input$tempsrefouinon)
        &amp;&amp;is.null(input$prodrefouinon)
        &amp;&amp;is.null(input$checkprod)
        &amp;&amp;is.null(input$checkprodref)){
       return()
     } else {
       data&lt;-filedata()
       res &lt;- tests.stats(data = data(),
                y = input$varinteret, 
                group = input$varprod, 
                group2 = input$vartemps, 
                sujet = input$varsujet, 
                apparie = input$apparie, 
                temoin = input$prodrefouinon, 
                TemoinName = input$checkprodref)
     }  
  </code></pre>
  
  <p>and the error message:</p>
  
  <pre><code>Warning: Error in &lt;-: invalid (NULL) left side of assignment
  Stack trace (innermost first):
      40: server [C:\Users\itm\Desktop\Documents\appli/server.R#357]
       1: shiny::runApp
  Error in outlist() &lt;- reactive({ : invalid (NULL) left side of assignment
  </code></pre>
  
  <p>Why am I receiving this error?</p>