How to add GH_Group object to the document

Hi,

There is a cluster with some input components inside a gh_group in one document and i try to read and add this group with there objects inside another document.
I can see that the group is already added (VS breakpoint), but nothing shows up in the gh_canvas.
If only the content of the group is added to the document it works.

foreach (IGH_DocumentObject docobj in docu.Objects)
{
    if (docobj is GH_Group gr)
    {
        if (!adoc.GetAllInstanceIDs(false).ContainsKey(gr.InstanceGuid))
        {
            adoc.AddObject(gr, false);
            foreach (IGH_DocumentObject grobj in gr.Objects())
            {
                adoc.AddObject(grobj, false);
            }                       
        }                        
    }
}