Hi, 2 issues I found with the mod:

Every time you press the manual save button, it creates a new window, so if you press it 10 times in a row, you have 10 windows instead of just having the same single window.
Second issue is the real problem. If you happen to save while having those 10 windows open and then load that save, you cant close the windows anymore. The event simple lost the reference to the frame and so it cant close it. On loading the game, the engine recreates existing frames but the button event was associated with a frame variable that does not exist anymore and so, doesnt work anymore.

Huh. That's an interesting failure mode, I'd not expected Factorio to attempt to save/load anything about window frames. I'll have a think about what approach I want to take to handle that.