Hello!
I've tried using your mod recently with my save (I was looking for that kind of mod for sooooo long).
Unfortunately, I get a crash the moment I click on the Railbow button, I don't know why... Here's the log:

Error while running event RailBow::on_gui_click (ID 1)
RailBow/scripts/gui_layout.lua:214: attempt to index local 'selected_tiles' (a nil value)
stack traceback:
RailBow/scripts/gui_layout.lua:214: in function 'add_choose_elem_table'
RailBow/scripts/gui_layout.lua:238: in function 'add_choose_elem_frame'
RailBow/scripts/gui_layout.lua:259: in function 'create_railbow_window'
RailBow/scripts/gui.lua:8: in function 'open_gui'
RailBow/scripts/gui.lua:25: in function 'handler'
core/lualib/event_handler.lua:47: in function <core/lualib/event_handler.lua:45>

Any idea on what I could do to avoid this? Hope this isn't too hard to fix

Hi!
Sorry to hear that, no idea how this might've happened. But it seems that somehow some game state data has been misconfigured.
Was the mod just freshly installed or did this happen after it was updated?
Anyway, I just pushed an update that should fix that, I hope. If you have any more issues do tell.

Hello.
The mod was freshly installed at the time when the error happened.
I've just tried updating it and playing with it, but unfortunately it still crashes anytime I press the Railbow button... With the exact same error log.

Error while running event RailBow::on_gui_click (ID 1)
RailBow/scripts/gui_layout.lua:214: attempt to index local 'selected_tiles' (a nil value)
stack traceback:
RailBow/scripts/gui_layout.lua:214: in function 'add_choose_elem_table'
RailBow/scripts/gui_layout.lua:238: in function 'add_choose_elem_frame'
RailBow/scripts/gui_layout.lua:259: in function 'create_railbow_window'
RailBow/scripts/gui.lua:8: in function 'open_gui'
RailBow/scripts/gui.lua:25: in function 'handler'
core/lualib/event_handler.lua:47: in function <core/lualib/event_handler.lua:45>

Could this be because of some other mod interacting with yours in a bad way?

Could you send over your save file please?

Sure, here (remove the quotes "):

">>>eNpjZGBkyGOAAHsGhgP2PCzJ+Yk5q1etsmNgaLAHYa7k/IKC1CLd
/KJUhDCDPWdyUWlKqm5+Jqri1LzU3ErdpMTiVJgQCHOk5+ekoJvAnVm
km5ZZXFxalFqMbAY/ULyoNCkJaGdJUWpqMYpBmUX5eegGsRaX5OelIh
vBXpKJqQrDMO7SosS8zNJcVJVAzOjm+eNNQ4scAwj/r2dQ+P8fhIGsB
0CTQJiBsQFIODAwAsVggDU5JzMtjYFBwZGBocAZaJAdIyNjtcg694dV
U+wZIWr0HKCMD1CRA0kwEU8Yw88Bp5QKjGGCZI4xGHxGYkAsLQFaAVX
F4YBgQCRbQJKMjL1vty74fmyDHeOflR8v+SYl2DMauoq8+2C0DuR2dp
A/meDErJkgsBPmFQaYmQ/soVI37RnPngGBN/aMrCAdIiDCwQJIHPBmZ
mAU4AOyFvQACQUZBpjT7GDGiDgwpoHBN5hPHsMYl+3R/QEMCBuQ4XIg
4gSIAFsIdxkjhOnQ78DoIA+TlUQoAeo3YkB2QwrChydh1h5Gsh/NIZg
RgewPNBEVByzRwAWyMAVOvGCGuwYYnhfYYTyH+Q6MzCAGSNUXoBiEB0
58UKMgtIADOLiZGRDggz3Dh9e+agBGlsaW<<<"

Is that a map exchange string? That doesn't help me, I would need your save file, to really see what's going wrong, because so far I've been unable to replicate your issue or pinpoint where it might be coming from.

Ah I see, you need the zip file. I apologize, I wasn't sure what to send or how to send it through this thread
You can download my save on mediafire here: https://www.mediafire.com/file/xtjxi7zpqrjfqcn/Railbow_IR3_Crash_Save.zip/file
Hopefully this helps.

That should fix it!
Still no clue how it happened.
Made a migration to fix your game state and others alike.
Put it in some safeguards so it doesn't happen again.

Thank you! It works like a charm now :D