Only happens when "quality" mod is enabled and quality is researched.

Crash when configuring combinators:
The mod Cybersyn Combinator (2.3.3) caused a non-recoverable error.
Please report this error to the mod author.

Error while running event cybersyn-combinator::cybersyn-combinator-confirm-gui (ID 201)
cybersyn-combinator/scripts/gui.lua:347: attempt to index field 'signal' (a nil value)
stack traceback:
cybersyn-combinator/scripts/gui.lua:347: in function 'set_new_signal_value'
cybersyn-combinator/scripts/gui.lua:627: in function 'confirm_signal_value'
cybersyn-combinator/scripts/gui.lua:2270: in function 'on_input_confirm'
cybersyn-combinator/scripts/gui.lua:2327: in function <cybersyn-combinator/scripts/gui.lua:2327>

Steps to reproduce:
- start new game
- research quality
- enable "Convert all item and fluid signal values to negative values" mod setting
- place down the combinator and open it
- add any request of any value to any slot
- select same slot
- right click it to remove it
- select same slot to add new request
- select same requested item
- confirm with "e" keybind

Only happens when:
- you have quality enabled (so you can confirm with "e")
- selected the request slot in step 6 before removing it in step 7 (to store something in the UI, because even after request is removed it still shows the previously requested values)
- confirming with "e" (since pressing the checkmark with a mouse work just fine)

Found the issue: The E key is able to (attempt to) confirm the signal value even when no signal is chosen yet (the signal picker dialog doesn't prevent the E key press from propagating down to the main window, sadly). I have a fix for this that will be released together with the "logistic section support" update.

Thanks for the report!

And this should now be fixed with the release of v2.4.0.