Error while running event Smart_Inserters::on_built_entity (ID 6)
real number expected got nil.
stack traceback:
[C]: in function 'destroy'
Smart_Inserters/scripts/world_selector.lua:66: in function 'update_positions'
Smart_Inserters/control.lua:601: in function <Smart_Inserters/control.lua:481>

Can you tell me which entity you were placing and a list of the mods/a save file so I can try to reproduce the problem?

I'm playing Ultracube: Age of Cube, and it provides an extremely long-handed filter inserter with a range of 5 tiles. However, I've set the max range of each inserter to 4. When I use the In-world drop/pickup position changer on it, the game crashes.

can you send me the settings that you used for Smart Inserters?

With the max range of each inserter set to 4 and disable long inserters unchecked, all other settings are left at their default values.
I believe the problem is that the "Extremely long hand filter inserter" has a range of 5 tiles. This exceeds the 4 tile setting I configured. I just tried changing the max range of each inserter to 5 and it didn't cause the game to crash.

The problem is not what you would expect actually, it simply tries to update the in_world selector but to do so it tries to delete the old sprite (sprite that does not exist since it exists till range 4 in your case) so it crushes.

"[C]: in function 'destroy'"

If you want to try the mod before release join the discord: https://discord.com/invite/753wmGJ
Ill push there an update before uploading it there.