Hey, love your mod, I am playing a biter heavy world with a lot of mods, no problems until I wanted to up the ante.
So I changed some settings until I got an error beginning the saved game. Isolated the change in settings that gave the problem: changing bitters from 6 tiers to more tiers (tried 8, 10 and 15, always same result).
By not changing the amount of tiers I could load the map, with for example the difficulty increased. When I saved the map and tried to increase the amount of tiers, I could open the map, and play the map, but as soon as I wanted to open the GUI of the mod, I got this error:

The mod Schall Endgame Evolution (0.18.4) caused a non-recoverable error.
Please report this error to the mod author.

Error while running event SchallEndgameEvolution::on_gui_click (ID 1)
SchallEndgameEvolution/lib/EEgui.lua:240: attempt to index field '?' (a nil value)
stack traceback:
SchallEndgameEvolution/lib/EEgui.lua:240: in function 'frame_main_refresh'
SchallEndgameEvolution/control.lua:32: in function <SchallEndgameEvolution/control.lua:20>

So, I'm increasing for 6 tiers to 15 tiers, evolution was quite high so immediatly the visual part of the GUI gave notice of tier 15 biters.
Thought this might be the problem, the sudden increase, so with control commands I changed evolution to 0.92, which corresponts to tier 6, yet again when I tried to open the GUI I got the error.

Can I provide you with additional information?

Can you please try the following steps, to see if it works?

1/ In the game mod folder, decompress the mod ZIP file SchallEndgameEvolution_0.18.4.zip.
2/ Edit lib/EEgui.lua, change line 330 from

EEgui.frame_main_refresh(player)

to

EEgui.frame_main_close(player)

3/ Load your game with changed highest tier.

I have made a proper fix in version 0.18.5.
Please use that and see if it works for you.

It's fixed, thanks!