title: mdMenu: Cannot read property 'open' of null
body: <p>Working with angular-material design, found a bug...
  On md-menu, if you have a sub menu item (as you can see in the image)and you open the subMenu and then hover over the non-subMenu item (menu item) it throws the error "Cannot read property 'open' of null" and remains highlighted even after hovering over a different menu items and need a click to open sub-menu-item.</p>
  
  <p>code:</p>
  
  <pre><code>&lt;md-menu&gt;
          &lt;button ng-click="$mdOpenMenu()"&gt;menu&lt;/button&gt;
          &lt;md-menu-content&gt;
            &lt;md-menu-item&gt;
              &lt;md-button ng-click="---"&gt;menu item&lt;/md-button&gt;
            &lt;/md-menu-item&gt;
            &lt;md-menu-divider&gt;&lt;/md-menu-divider&gt;
            &lt;md-menu-item&gt;
              &lt;md-menu&gt;
                &lt;md-button ng-click="$mdOpenMenu()"&gt;sub menu&lt;/md-button&gt;
                &lt;md-menu-content&gt;
                  &lt;md-menu-item&gt;&lt;md-button ng-click="---"&gt;sub menu item&lt;/md-button&gt;&lt;/md-menu-item&gt;
                &lt;/md-menu-content&gt;
              &lt;/md-menu&gt;
            &lt;/md-menu-item&gt;
          &lt;/md-menu-content&gt;
        &lt;/md-menu&gt;   
  </code></pre>
  
  <p><a href="https://i.stack.imgur.com/wYoLh.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/wYoLh.png" alt="Error get when i hover on menu item from sub menu item..[![][1]"></a>]<a href="https://i.stack.imgur.com/Ra0NU.png" rel="nofollow noreferrer">2</a></p>