:type: list of ProtocolTreeItem

def dropEvent(self, event: QDropEvent):
        items = [item for item in self.items(event.scenePos()) if isinstance(item, GraphicsItem) and item.acceptDrops()]
        item = None if len(items) == 0 else items[0]
        if len(event.mimeData().urls()) > 0:
            self.files_dropped.emit(event.mimeData().urls())

        indexes = list(event.mimeData().text().split("/")[:-1])

        group_nodes = []
        file_nodes = []
        for index in indexes:
            try:
                row, column, parent = map(int, index.split(","))
                if parent == -1:
                    parent = self.tree_root_item
                else:
                    parent = self.tree_root_item.child(parent)
                node = parent.child(row)
                if node.is_group:
                    group_nodes.append(node)
                else:
                    file_nodes.append(node)
            except ValueError:
                continue

        # Which Nodes to add?
        nodes_to_add = []
        """:type: list of ProtocolTreeItem """
        for group_node in group_nodes:
            nodes_to_add.extend(group_node.children)
        nodes_to_add.extend([file_node for file_node in file_nodes if file_node not in nodes_to_add])
        protocols_to_add = [node.protocol for node in nodes_to_add]

        ref_item = item
        position = None if ref_item is None else item.drop_indicator_position
        self.add_protocols(ref_item, position, protocols_to_add)
        super().dropEvent(event)