title: Can't get Edge to Return a Promise in my WebExtension
body: <p>I am trying to create a new tab using <code>browser.tabs.create()</code> from my <code>background.js</code> WebExtension as shown here:</p>
  
  <pre><code>createTab: function () {
      var newTab = browser.tabs.create({ url: someUrl });
      newTab.then(onCreated, onError);
  }
  </code></pre>
  
  <p><strong>The new tab creates in the browser</strong>, but when the last line is reached, an error is thrown:</p>
  
  <blockquote>
    <p>SCRIPT5007: Unable to get property 'then' of undefined or null reference</p>
  </blockquote>
  
  <p>The Locals window shows <code>newTab</code> is <code>undefined</code>.  </p>
  
  <p>What am I doing wrong here? I thought that <code>.create()</code> would immediately return a <code>Promise</code>. I know <code>create()</code> is an asynchronous function - but my calling function doesn't need to by async, does it?  </p>
  
  <p>Any help would be appreciated.  </p>