title: Apps Script Sidebar Reference Error When Using Include
body: <p>I created a <code>Google Apps Script</code> add-on to display a sidebar on click:</p>
  
  <pre><code>function displayPage_() {
    getScriptProperties_()
    var htmlTemplate = HtmlService.createTemplateFromFile('PAGE');
    var html = htmlTemplate.evaluate()
            .setTitle('Connector');
    SpreadsheetApp.getUi().showSidebar(html);
  }   
  </code></pre>
  
  <p>This successfully loads the file <code>PAGE.html</code> as long as it is simple html. However, when I try to add <code>include</code> like this:</p>
  
  <pre><code>&lt;!DOCTYPE html&gt;
  &lt;html&gt;
    &lt;head&gt;
      &lt;base target="_top"&gt;
      &lt;?!= include("STYLESHEET"); ?&gt;
    &lt;/head&gt;
    &lt;body&gt;
    &lt;/body&gt;
  &lt;/html&gt;
  </code></pre>
  
  <p>It throws this error:</p>
  
  <pre><code>[17-02-09 08:55:50:239 MST] Execution failed: ReferenceError: "include" is not defined.
  </code></pre>
  
  <p>It doesn't matter what is in the <code>include</code> it always fails.</p>
  
  <p>I have done this before and it worked. As far as I can tell, I have this set up just like the other project, but it doesn't work in this project. I assume that I forgot to enable something, but don't know how to tell what is missing because the transcript is vague.</p>
  
  <p>What am I missing? Or, doing wrong?</p>