Error while running event GUI_Unifyer_Unified::on_gui_closed (ID 97)
GUI_Unifyer_Unified/control.lua:1086: attempt to index global 'global' (a nil value)
stack traceback:
GUI_Unifyer_Unified/control.lua:1086: in function <GUI_Unifyer_Unified/control.lua:1081>

I've implemented a fix for them in the latest release 2.0.1, can you please try if this fixes it for you? If the issue still occurs, enable logging in the mod's settings under 'per player' and set the Log level to DEBUG. After that, provide the Factorio Log file. That should give me a better idea on what's causing the crashes.

Hopefully it shouldn't happen again though...

With the latest update, if I load a save that has a window open in it already (like if you have the electric network window up and an autosave happens), and try to close the window, I get this crash:

The mod GUI Unifier (2.0.1) caused a non-recoverable error.
Please report this error to the mod author.

Error while running event GUI_Unifyer::on_gui_closed (ID 99)
GUI_Unifyer/control.lua:1117: attempt to index global 'global' (a nil value)
stack traceback:
GUI_Unifyer/control.lua:1117: in function <GUI_Unifyer/control.lua:1105>

it seems to be fine if I load a save without a window open already though. I turned on debug logging and saved the log if you need it

So this crash happened a few more times to me and it didn't matter if I had a window opened or not when I saved, but it seemed to only happen with auto saves.

I fixed it by going into control.lua and did a find/replace on "global" to "storage" (I think it changed 103 globals) and it seems to be working. I haven't had a crash since I did that.
Also as a side note I had to do the same for the "unified" version of this mod.