Found another crash, when I was updating one of my LTN(Logistic Train Network) Train stops:
https://imgur.com/6X6RgS8
Seems to occur when updating any blueprint, tried one with just chests and just this mod, doesn't appear to be related to LTN.

Hi achman741. The issue should now be fixed in 1.0.8.

Not quite. This happens when I copy a piece of a belt. That is, I just press CTRL+C, drag a frame around a belt, and release mouse button, and get this error.

1706.111 Error MainLoop.cpp:1207: Exception at tick 16629034: Мод Improved Combinator (1.0.8) вызвал неустранимую ошибку.
Пожалуйста, сообщите об этой ошибке автору мода.

Error while running event ImprovedCombinator::on_player_setup_blueprint (ID 68)
ImprovedCombinator/scripts/entity.lua:175: attempt to index local 'entity' (a nil value)
stack traceback:
ImprovedCombinator/scripts/entity.lua:175: in function <ImprovedCombinator/scripts/entity.lua:158>
1706.111 Error ServerMultiplayerManager.cpp:90: MultiplayerManager failed: "Мод Improved Combinator (1.0.8) вызвал неустранимую ошибку.
Пожалуйста, сообщите об этой ошибке автору мода.

Error while running event ImprovedCombinator::on_player_setup_blueprint (ID 68)
ImprovedCombinator/scripts/entity.lua:175: attempt to index local 'entity' (a nil value)
stack traceback:
ImprovedCombinator/scripts/entity.lua:175: in function <ImprovedCombinator/scripts/entity.lua:158>"

I think the issue you are describing might be different one.
Thanks to your stack trace, I was able to reproduce it on a multi-player game. I've added several checks to prevent this issue from happening.

It "should" be fixed in 1.0.9.

tyvm