import c4d

def main():
    mapping = {}
    context = doc.GetActiveObject() or doc.GetFirstObject()

    if not context:
        return

    root = context.GetUp()

    # Go to the first object in the context.
    while context.GetPred():
        context = context.GetPred()

    # Fill the mapping.
    while context:
        mapping.setdefault(context.GetName(), []).append(context)
        context = context.GetNext()

    # Create Null-Objects.
    items = sorted(mapping.iteritems(), key=lambda x: x[0])
    prev = None
    for name, objects in items:
        null = c4d.BaseObject(c4d.Onull)
        null.SetName(name)
        for obj in objects:
            doc.AddUndo(c4d.UNDOTYPE_CHANGE, obj)
            obj.Remove()
            obj.InsertUnderLast(null)
        doc.AddUndo(c4d.UNDOTYPE_NEW, null)
        doc.InsertObject(null, root, prev)
        prev = null

    c4d.EventAdd()

main()