title: Error when using add_submenu_page in a different plugin to add_menu_page
body: <p>I am trying to develop multiple plugins to fill various needs I have in WordPress. I figured I would create a 'base' plugin, and then other plugins can be installed on top.</p>
  
  <p>I have created a menu page in my base plugin using add_menu_page (all this is in my class obviously, variables replaced with strings for ease of reading)</p>
  
  <pre><code>private function __construct() {
      add_action('admin_menu', array($this, 'add_group_page'));
  }
  
  public function add_group_page(){
      add_menu_page('Group Plugins', 'Group Plugins', 'activate_plugins', 'group-plugins', array($this, 'group_page_callback'), '', 3.1415);
  }
  
  public function group_page_callback() { ?&gt;
      &lt;div class="wrap"&gt;
          &lt;h2&gt;Group Plugins&lt;/h2&gt;
      &lt;/div&gt;
  &lt;?php
  }
  </code></pre>
  
  <p>That works fine, my menu structure now has: (tags wrong on purpose, for readability of urls)</p>
  
  <p>(Group Plugins)[<code>admin.php?page=group-plugins</code>]</p>
  
  <p>In my second plugin, I try to call <code>add_submenu_page</code>:</p>
  
  <pre><code>private function __construct() {
      add_action('admin_menu', array($this, 'add_groupforms_page'));
  }
  
  public function add_groupforms_page(){
      add_submenu_page('group-plugins', 'Group Forms', 'Group Forms', 'manage_options', 'group-forms', array($this, 'groupforms_page_callback'));
  }
  
  public function groupforms_page_callback() { ?&gt;
      &lt;div class="wrap"&gt;
          &lt;h2&gt;Group Forms&lt;/h2&gt;
      &lt;/div&gt;
  &lt;?php
  }
  </code></pre>
  
  <p>Which <em>seems</em> like it has worked in adding the new submenu item. But the menu structure changes to:</p>
  
  <p>(Group Plugins)[<code>admin.php?page=**group-forms**</code>]</p>
  
  <p>--(Group Forms)[<code>admin.php?page=**group-forms**</code>]</p>
  
  <p>So both links link to the new submenu page, and when I click on either of them, I just get a backend styled page that says <code>You do not have sufficient permissions to access this page.</code></p>
  
  <p>I can still access 'Group Plugins' original page if I change the slug in the url to '<code>group-plugins</code>', but something clearly isn't right, and the only thing that really shows up when I google the error is people saying to check its hooked in the right place: <code>admin_menu</code>, which it is, because I have checked the docs so many times trying to figure this out.</p>