The Main Object is not part of any other group

The line

group = chart.CreateGroup(nodes[0]); 

in the code below returns null, so the LoadGroup function throws a 'null' exception

  public void CreateGroup(List<Node> nodes, GroupTypes GroupType)
        {
            Group group;
            if (groups.ContainsKey(GroupType))
                groups.Remove(GroupType);
            if (nodes.Count > 0)
            {
                group = chart.CreateGroup(nodes[0]);
                LoadGroup(group, nodes);
                groups.Add(GroupType, group);
            }
        }


  private void LoadGroup(Group group, List<Node> allnodes)
        {
            foreach (Node node in allnodes)
            {
                if (node != group.MainObject)
                    group.AttachToSideMiddle(node, (int)ArrowAnchors.Left);
            }
        }
 

This is happening when I nest groups, whereby a main object of one group will become a subordinate in a parent group.

I do not, however, have an occasion where an object will be used as a main object or a subordinate more than once

Very frustrating, any ideas?
Cheers
JC

A console.write revealed that I was, unintentionally, trying to set a group master with an object already a group master of another group.

Thanks for letting me waste your time