CreateDrag&Drop node takes wrong wariable type from DragDropOperation class

I have drag and drop operation in my inventoty widget. When i selecting a DragDropOperation class in CreateDrag&Drop node, everything works fine:

But, when i reopen the project, CreateDrag&Drop node change its DragOffset variable type and error appears:

I can fix the error by choosing “None” in CreateDrag&DropOperation class and then choose my DragDropOperation class again. But i must do it everytime i reopen the project.

Is anybody know how to fix the problem permanently?