def _on_item_expanded(self, index) -> None:
        """
        Descend into call tree for this node.
        """
        expanding_item = self._model.itemFromIndex(index)
        if not expanding_item.populated:
            dbg = self.instance.debugger_mgr.debugger
            if dbg.am_none:
                return
            called = dbg.get_called_functions(expanding_item.event)
            for func_or_addr, event in called:
                expanding_item.appendRow(CallTreeItem(func_or_addr, event))
            expanding_item.expandable = len(called) > 0
            expanding_item.populated = True

        Descend into call tree for this node.
        
angrmanagement/ui/views/call_explorer_view.py