The mod Smart Inserters (1.2.9) caused a non-recoverable error.
Please report this error to the mod author.

Error while running event Smart_Inserters::on_gui_click (ID 1)
Smart_Inserters/scripts/inserter_functions.lua:325: attempt to index local 'inserter' (a nil value)
stack traceback:
Smart_Inserters/scripts/inserter_functions.lua:325: in function 'get_inserter_orientation'
Smart_Inserters/scripts/selector_gui.lua:423: in function 'on_button_position'
Smart_Inserters/control.lua:216: in function <Smart_Inserters/control.lua:208>

Ill check then report what I found here

Host and play with a friend, dosent happen often but always when clicking a UI button to change the inserter pickup / drop position.
Only config change is to disable range tech.

Does it happen when clicking the button or when you miss it slightly?
Also, can you tell me if this happens on any inserter or only a specific mod/entity?
In case could you send me a save and a screenshot of all the mods that affect inserters?

World: https://www.dropbox.com/scl/fi/bws0sp94aqqtpwszmam0d/Normal_World.zip?rlkey=mvgy8v5u9q1xcjc5qmflja3az&dl=0

Inserter: https://www.dropbox.com/scl/fi/zzny5c9x7urutghf9ayod/chrome_hZpwN6s1Vi.png?rlkey=akhlog0ers0qt1pv09cj9qf1h&dl=0

As for clicking it or miss clicking it, attempting to click every pixel around and on the buttons seems no immediate issue, as its only happend about 5-6 times in this 82 hour save so far which makes it a 'fun' bug to reproduce.

Specific inserter mainly on fast inserters, but has happend at least once on a long inserter.

It's quite hard to reproduce, I added a check in the function that crashes in the bug report you sent me but I could not figure out how that is possible, basically the opened entity is not the inserter you want to edit, maybe some mods change your focus but I really have no idea how/when.

If the bugs persist I'll do some more investigations.