Hi everyone.
I use the WBS Gantt Chart App to make Gantt charts on my projects. Once I finish the planning, I save it and exit the Gantt. When I enter again in the same Gantt, however, I get the message reported below.

Can you tell me why? What does it mean? What happens to my planning if I press the Undo button (apparently nothing)?
In particular, I would like to know if I am doing something wrong in the way I use the app or in the way I set the settings.
Thank you very much to those who will help me!