Sad to say, the error still occurs in 3.0.2. :/

Getting crazy over this. Would really need to upgrade to 3.0.2 but this problem makes it impossible. Tried again with a fresh install and db with the code that IOTI wrote, but I still get the singleton error. Also get the same error when I try to add a member trough the autocompleter field.

In the singleton function in core.php, I added a "echo $className" right in the beginning. The output contained 1 "Member" more in the 3.0 installation then in later. Don't know exactly what it means, but it should mean something, right?

If it something I do wrong, what could it be? I have no idea...

Update. If I comment line 186 out in Group.php it works as it should. The line reads:

$fields->push($idField = new HiddenField("ID"));

I changed it to a textfield and its value is set in all versions (3.0.X). Havn't done any estensive testing yet but I don't se any side effects so far.

So the question is, what does this line do and why is it messing with me?! To be continued...

Old post, but I just encountered this.
Looks like the "ID" field name is in use, you can get around it by renaming that field.

Hi,

I'm using the 3.1.2 version and was still looking for a solution. Finally I managed to find it.

I followed the stack trace and discovered that LeftAndMain.php was trying to retrieve the current page (currentPage) by calling

$this->getRecord()

The first thing that getRecord does is to detect the className property of the extension object.
This is done by querying the variable $site_tree

        public function getRecord($id) {
                $className = $this->stat('tree_class');
.
.
.

So I added $site_tree to my extension manager class:

class GroupModelAdmin extends ModelAdmin {

   public  static $tree_class = 'Group';
   public static $managed_models=array(
       'Group'
   );

   public static $url_segment='groupManager';
   public static $menu_title='Group Manager';

}

And that solved the problem....