1. Enter artillery wagon.
2. Click "disable artillery". Get kicked from wagon during entity replacement, but Artillery Switch GUI does not disappear.
3. Click "enable artillery", and get the exception below:

4302.177 Error MainLoop.cpp:1142: Exception at tick 1192230: The mod Artillery Switch caused a non-recoverable error.
Please report this error to the mod author.

Error while running event ArtillerySwitch::on_gui_click (ID 1)
ArtillerySwitch/control.lua:125: attempt to index field 'vehicle' (a nil value)
stack traceback:
ArtillerySwitch/control.lua:125: in function <ArtillerySwitch/control.lua:113>

Thanks for reporting this, I thought I fixed this before but something must have changed. The mod is updated with the fix.