title: jQuery slideDown ($.slideDown) animated error: Uncaught TypeError: Cannot read property 'style' of undefined
body: <p>This simple code not working:</p>
  
  <pre><code>$('li a').off('click').on('click', function(e){
    var checkElement = $this.next();
    checkElement.slideDown(500, function () {
          checkElement.addClass('menu-open');
    });
  });
  </code></pre>
  
  <p>HTML:</p>
  
  <pre><code>&lt;ul class="sidebar-menu"&gt;
  &lt;li&gt;
      &lt;a href="unsafe:javascript:void(0)"&gt;
          &lt;i class="fa "&gt;&lt;/i&gt;&lt;span class="ng-binding"&gt;Level 1&lt;/span&gt;
          &lt;span class="pull-right-container"&gt;
              &lt;i class="fa fa-angle-left pull-right"&gt;&lt;/i&gt;
          &lt;/span&gt;
      &lt;/a&gt;
  
      &lt;ul class="treeview-menu"&gt;
          &lt;li&gt;
              &lt;a href="#/level2" class="ng-scope"&gt;
                  &lt;i class="fa "&gt;&lt;/i&gt; &lt;span class="ng-binding"&gt;Level 2.1&lt;/span&gt;
                  &lt;span class="pull-right-container"&gt;
                      &lt;i class="fa fa-angle-left pull-right"&gt;&lt;/i&gt;
                  &lt;/span&gt;
              &lt;/a&gt;
          &lt;/li&gt;
          &lt;li&gt;
              &lt;a href="#/level2" class="ng-scope"&gt;
                  &lt;i class="fa "&gt;&lt;/i&gt; &lt;span class="ng-binding"&gt;Level 2.2&lt;/span&gt;
                  &lt;span class="pull-right-container"&gt;
                      &lt;i class="fa fa-angle-left pull-right"&gt;&lt;/i&gt;
                  &lt;/span&gt;
              &lt;/a&gt;
          &lt;/li&gt;
      &lt;/ul&gt;
  &lt;/li&gt;
  &lt;/ul&gt;
  </code></pre>
  
  <p>Debuging: <code>checkElement</code> in <code>var checkElement = $this.next();</code> It contains an array of a position with the 'ul' element sibling:
  <a href="https://i.stack.imgur.com/vfwMA.png" rel="nofollow noreferrer">print Chrome debugger</a></p>
  
  <p>Error:</p>
  
  <blockquote>
    <p>jquery-2.2.3.min.js: Uncaught TypeError: Cannot read property 'style' of undefined</p>
  </blockquote>
  
  <p>The exception is thrown before executing the callback / complete function.
  In jQuery the error is within the method: animate</p>
  
  <p>I also have the Angular loaded.</p>
  
  <p>Forgive me for bad English</p>